Depends on what kind and density # powder you are using, depends on what brand of caps you use, it also depends on what type of grease you use and how well you keep it clean, it sounds complicated, but it's not, you just need to get some good instruction, either from someone who knows what there doing or from alot of reading.
I shoot a pair of 1851 Navys and a pair of 1860 Armys in the cowboy shoots, I can usually get off fifty rounds out of each with no problem, with nothing more than a quick wipe down in-between relays.
It's like everything else, you only get out of something what your willing to put into it, if you stick with good guns like Pietta, or Uberti, use quality products, keep them clean, and get the proper instruction, you will have a fun time with them, if you don't do it right, it will be a pain in the butt every time you shoot them.
Remember, most of the cap and ball guns on the market today are every bit the equal or better than the originals were, they are accurate, powerful, and if not treated safely they are dangerous, have fun with them, but do it safely.