Depends on some decisions based upon tiers of play. If there will be an apprentice tier, then I can see a lower maximum, like 5 or 6, if heroic is where we are going to start, then 10 or 11 is a good starting point.
Maybe something like
+6 at level 1 of apprentice tier. (gain about +1 per apprentice level)
+11, at level 1 of heroic tier. (gain about +0.5 per heroic level)
+16, at level 1 of paragon tier. (gain about +0.5 per paragon level)
+21, at level 1 of epic tier. (gain about +1 per epic level)
And these would be maximums, including a "bonus". I am against stacking as many bonuses as 4e has. I don't want to see +2 from background, +2 from race, +2 from theme, +2 from class feature/power/stance, +2 from item, +2 from paragon path, +2 from epic destiny, creating a discrepancy of 10+ points between two max stat trained characters. I would like to see a non-stacking rule for skill bonuses. If you get a racial bonus, you can't also have a background/theme bonus to the same skill. Pick a different skill. Or I might be fine with mechanics that maybe let you reroll, or roll two dice, on a skill you already have a bonus in, possibly with some conditions/caveats.
Maybe something like
+6 at level 1 of apprentice tier. (gain about +1 per apprentice level)
+11, at level 1 of heroic tier. (gain about +0.5 per heroic level)
+16, at level 1 of paragon tier. (gain about +0.5 per paragon level)
+21, at level 1 of epic tier. (gain about +1 per epic level)
And these would be maximums, including a "bonus". I am against stacking as many bonuses as 4e has. I don't want to see +2 from background, +2 from race, +2 from theme, +2 from class feature/power/stance, +2 from item, +2 from paragon path, +2 from epic destiny, creating a discrepancy of 10+ points between two max stat trained characters. I would like to see a non-stacking rule for skill bonuses. If you get a racial bonus, you can't also have a background/theme bonus to the same skill. Pick a different skill. Or I might be fine with mechanics that maybe let you reroll, or roll two dice, on a skill you already have a bonus in, possibly with some conditions/caveats.