Don't know percentages, etc. as Rob (game dev) doesn't really share that data with us, but also because there are so many variables. Some basic mechanics are:
> The closer a player is to their potential, the slower they will train and the faster they will decline
> Younger players train faster and decline slower
> Players decline when they don't play or get coached, regardless of their age. The rate until the age of 24/25 is apparently negligible, then it really picks up. This applies to bowling speed to - your seamers get slower if they aren't bowling or working on pace.
> When you coach a player in a skill, the benefit they gain is offset slightly by losses in the skills they are not coaching in. Training Attack takes away from Defense, training Movement takes away from Variation, training L&L takes away from Pace, and vice versa. I think Training Shot Selection takes away slightly from both Attack and Defense, but not sure. The net result of a coaching session is always a positive, regardless of the skill level of the coach.
> All skills are not linear in terms of effectiveness. Going from 4 to 5 is a bigger jump compared to 13 to 14. This applies to coaches ability as well.
> Coaching only kicks in during Thursday coaching updates. Whatever you have set then is what gets trained. You can change it over the week, it has no effect. The game just sees what's set when it is time to run the update. Think of it as a single net session on a Thursday.
> The In-Game Training amounts vary by Division (higher Divisions offer more training), with Dev games offering more training than OD, which offers more than T20s.
> Bowling 1 over = receiving 50% of IGT on offer. Each over after that has diminishing returns, and bowling the full quota of overs = 100% of IGT.
> Batting even just 1 ball gets you 100% of the batting IGT on offer. Batting longer doesn't help. Not batting = 0 IGT.
> Being on the field for even 1 ball gets you 100% of the fielding IGT on offer. That means all 22 players get 100% of the fielding/wicketkeeping IGT regardless of how the game goes.
> Apparently the amount of IGT a player can get in a week is capped, to the point where playing them in 2 games is usually 'sufficient' - but the jury is out on this. Best advice is still to give your best prospects as much game time as possible.
> Similarly the rate of decline is such that a 25+ year old playing 2 games a week should be able to maintain his skills into his 30s. But if they're only playing 1 game a week the decline becomes more noticeable.