You'd be surprised at how long it might take to get to Epic levels using just the prior history. Assuming that each term of service gives you something like 6000xp, and it takes 32 terms to reach 20th level, and 35 terms to make it to 21st.
The game is set up to take about four adventures a level before advancement occurs, so it'd take four months (assuming one completed adventure a month) to bring a character up a level. Assuming that the characters generated out in prior history to a max of 10th level (which is the highest I've seen rolled in my games), that's still almost three years of gameplay before you hit Epic Levels. That sounds great!
Now, of course, other gamers have other experiences with advancement, etc, so it might be sooner or later than that.
Once a character makes 21st level, I'd suggest using the basic Epic rules as available in the SRD. For taking class levels over 20th, I'd suggest another class feat every three levels, build some Epic Feats if you need them, and let it go at that.
Hope this helps,
Flynn