I'm playing a Tremere maga in the Theban Tribunal and I'm working on a few things involving Gigas, her home covenant. I'm sure it's kept vague in True Lineages on purpose, but I'm try to get a handle on the 'life cycle' of the Tremere; how long after Gauntlet is it before a young magus is considered to be Skilled or Experienced? I'm sure it would vary a lot on a case by case basis, but I'm wondering if anyone else has given this sort of thing any thought.
Both from the average time it would take to win someone's sigil, and from the average time it takes Rhine magi to become masters (which is basically something that marks them as experienced, or at least skilled magi), I'd say that the "average" Tremere could be considered "skilled" within one or two decades after Gauntlet, and "experienced" after another three decades (for a total of thirty-fifty years of post gauntlet experience). Ultimately, it's not so much an issue of seniority as of accomplishment.
I was going to say 15 years and then another 30 years, so my intuition matches ezzelino's pretty well.
I'd have gone for 20 years and... rather later, due to the extended life spans of magi.
Actually a good rule-of-thumb for troups playing in the Rhine might be "When he's ready to be a master."
When it suits your game and your version of the Tremere, according ot your troupe.
That is, there's no fixed number, and deliberately so.
I always assumed that to be the case, I was just looking for a bit of inspiration 
I think I'm going to go with around 20-25 years for Skilled with an additional 45-50 years afterwards for Experienced; I'm assuming Experienced magi are probably eligible to become Tribunes, so they would have to have a quite a few years under their belts!
Note that according to HoH:TL an "experienced" magus is simply one who has won his sigil.
Assuming that it takes 65-75 years before a Tremere earns his sigil seems a little on the long side.
Assume your average magus lives 120 years after gauntlet (as is implied in several canonical sources).
Then, we are looking at two basic types of scenarios.
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A Tremere's parens is definitely older than his filius, 40-120 years older. In this case, the filius will get his sigil on the parens' demise, which is 0-80 years away. This means an average of about 40 years of waiting.
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A Tremere's parens is not that much older than his filius -- 15-40 years older. In this case, their magical puissance is sufficiently close that within 40 years the filius is more likely than not to have won his sigil. This is due to basically two factors. First, in any duel either party has a fair chance of botching (this is even stated in the RAW), so even the magically weaker party will eventually win a duel (in 40 years, in addition to the gauntlet's Certamen, a magus can challenge his parens 5 times). Second, Arts for Certamen exhibit something of a "rock, paper, scissors" effect, whereby any combination of Art scores is particularly effective against some combinations, and particularly weak against others; thus, by studying carefully, a filius has a fair chance of outmatching his parens even with less overall experience. My troupe and I have explored this quite a bit in our saga, since one of the PCs was a Tremere focusing on certamen duels.
My understanding has always been that the number of Tremere magi who have there own sigils is quite small; I've never tried to put a number to it before, but I guess somewhere around 20 or 30 is where I've always pictured it. By those numbers, i feel my age brackets are about right, but obviously that's only my take on it.
Both of these seem reasonable; compounding things in our game is that fact that her parens is Boustaphan, who has had several apprentices (I think I've placed my maga as his third or fourth, and he's got another one in the pipeline) since he's determined to achieve immortality through his line and is I think around 120. In a sense, both scenarios you've described might have occurred between him and his various fili.
I always like to consider a magus no longer in their spring season at around 25 years post-Gauntlet. I'd equate this to no longer being inexperienced for House Tremere, but it could vary depending on many factors.
The way I run Tremere is for the first 10-15 years of a new Tremere magus' life, their service-to-house often involves studying arts, abilities or spells dictated by their superiors to ensure they meet the requirements of the role they are being groomed for - often at the feet of a skilled teacher. For the next 10-15 years, they're still new but no longer a complete scrub. As such, service to the house is going to start looking like actual work. This means that a Tremere magus is likely to develop slightly faster for the first ~15 years, then slightly slower for the remainder of their life.
As for claiming a sigil... what I did was work out a really rough algorithm for providing arts scores for a magus based on their Hermetic age. For a Tremere magus' master, I simply plug those in and work out their strong/weak arts. It's then up to the junior magus to decide when to challenge - and the senior magus is not going to hold back!
Due to the scaling nature of arts, it's reasonable for the junior magus to have some arts exceeding that of their master and many others within a 3-5 point difference when they're around 50-ish years out of apprenticeship - though it depends on the age of their master. Certainly having a reasonable chance of victory at ~50 years post-Gauntlet is not out of the question.
So it seems my numbers are fairly similar to those of many others.