Starting a new topic since this was pulling another one off topic quickly...
Just how much bonus does an Affinity provide on average? While some say 50%, this requires ignoring the rule about rounding. Some claim using the rounding is gaming the system. To those critics, I have two points. First, that is how the rules work and I only want to base this on the actual rules. Second, even if you heavily game the system with and Affinity, the Affinity will still not nearly provide the benefit of Virtues like Book Learner. Book Learner essentially gives you half an Affinity for any Ability/Art when you learn from a book, which is where a large portion of a magus's experience comes from. Let's say the magus earns 1/3 of his experience from books. Then, roughly, the magus gets an extra 1/4 (+3 is roughly a quarter of a typical Quality) of 1/3 of his experience - an extra 1/12 of his experience - in every Art and many Abilities. So Book Learner provides 1/6 of the Affinity where you would have placed the Affinity, plus 1/6 an Affinity in every other Art and many other Abilities. Overall, even accounting for a large portion of experience being placed in the Art/Ability with the Affinity, Book Learner is much more valuable than an Affinity. Thus even heavily gaming Affinity keeps its value under that of some other Minor Virtues and so shouldn't be unacceptable.
All this being said, let's see what this rounding really does.
The first question is how many sources are odd? There are several sources where the Quality is effectively random. We have books, teaching/training, realia, Vis study. Meanwhile exposure, adventure, and practice can be split up as desired and so an odd source Quality can be guaranteed. Due to the separation of practice and Vis study the value shifts a little between Abilities and Arts. For now I'm going to assume we're dealing with Arts, recognizing and Affinity with an Ability will be even better. I'm also going to ignore the potential use of correspondence to make random odd Qualities become non-random.
The second question is what fraction of the magus's seasons are spent gaining experience in any one of these ways? This is highly saga dependent. However, I think we can make some reasonable estimates. I've played in many sagas with many different players and SG's, and they've all been reasonably similar in this regard. Teaching/training, Vis study, and realia tend to be used far less often than books. Since those are all random, too, we can lump them in with books for consideration here. Meanwhile, I would estimate a magus spends about 1 season per year adventuring. Based upon service to covenants and their own need to develop spells and items, there seems to be a balance between seasons in the lab and seasons spent studying. So as a rough estimate a year consists of 1 season of adventure, 1.5 seasons of lab work, and 1.5 seasons of study. Variations here will of course skew the numbers, but this should provide a good starting point.
The third question is how much experience is earned in a typical Art in a season? For the adventure you're limited to 5, so you're best off choosing 1, 3, or 5. On average that is 3 experience placed in the Art for 5 experience after the Affinity. For the lab work you're best off choosing 1 or 3 (correspondence can push the season of exposure from 2 to 3). On average that is 2 experience placed in the Art for 3.5 experience after the Affinity. For books I believe 10 to 11 is typical of Qualities, especially since you probably won't spend much time with a low-level summa for an Art in which you have an Affinity. On average that is 10.5 experience placed in the Art for 16 experience after the Affinity.
Now let's put that all together. (1x5+1.5x3.5+1.5x16)/(1x3+1.5x2+1.5x10.5)=34.25/21.75=1.575. So the Affinity gives +57.5% on average to an Art.
Just to take a quick look at the Ability and practice side of things, many Abilities that are practiced are usually either trained, taught, or practiced. If you have an Affinity, trainers/teachers become scarce much more quickly, so practice becomes a larger part. Also, grogs and the like rarely do correspondence, so I'll drop that from exposure. I'll assume ratios of 1 adventure : 1.5 exposure : 1 practice : 0.5 training. Practice usually doesn't provide Qualities above 4, though there are a bunch of instances of 5 or more. I'll leave 4 as the cap for now, though. You're best off choosing 1 or 3. So on average that is 2 experience placed in the Ability for 3.5 experience after the Affinity.
Now let's put that all together. (1x5+1.5x2+1x3.5+0.5x16)/(1x3+1.5x1+1x2+0.5x10.5)=19.5/11.75=1.660. So the Affinity gives +66.0% on average to such an Ability.
So it seems like an overall average somewhere around +60% is pretty accurate. Sure, you could see this drop as low as +50% or as high as +100%, but those are extreme cases. Also, do note that I did not include apprenticeship/creation since this can skew things heavily in either direction depending on which character creation method you use and depending on how much is placed in the given Art/Ability during apprenticeship/creation.