Are you asking for the maximum I would use or what the cap in the DMG should be?
For the former probably +1 or +2 if I use them. For the latter there should be no cap.
Max that you would use if you went all the way to max level--assuming 20 for that in this poll. Naturally, if you tend to stop well shy of maximum level, you'll probably avoid the upper end of the scale. Though assuming a more or less equal distribution, this has implications for every level.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.