The FCC has limited the maximum connection rate at 53kb in the U.S. You will generally connect at 40 - 50kb but that can be affected by many different factors. The maximum speed for dial-up is mainly based on the modem. Most people have a 56kb modem though, which is the fastest modem available for dial-up.

Many factors depend the fastest speed, but the absolute fastest speed possible for 56k modem is 53Kb or also around 6.6KB/second. Upload speed is capped at 33.6Kb or also 4.2KB a second. These are under best conditions though, which no one usually ever has.

As well as having ISP standards (which you should check on the dial-up numbers to see if it supports V.90, which most new modems come with) it also could be a problem with your phone lines. Pick up the phone and press 1, listen for static, if you hear any then it is most likely causing the slow speed.