PHB2's character generation guidelines give some ideas of instant equipment for a PC or NPC of this or that level.
For an NPC Cleric, Fighter, Knight, Marshal or Paladin, the +1 weapon appears at level 5, +2 weapon at 11th level, +3 weapon at 13th level, +4 at 15th level, +5 at 17th, +6 at 19th, +7 at 20th. This is far from being the only magical item listed in this automatic equipment. The +1 Full Plate shows up at 6th level, for instance, and becomes +2 at 8th, +3 at 12th, +4 at 14th, +5 at 18th, +6 at 19th.
Just to give an example.
These of course are guidelines. I handed out a +2 dagger and other more powerful magical items to our 4th level PCs last session because I knew that for the last few sessions that remain for them to play, there would be a LOT of damage going on, and ultimately, they won't be there to play next year so... that's just a power increment for the next few sessions. Believe it or not, there's more... much more... coming their way.