I've been attending regional theatre for many years, and have seem plays ranging from about 75 minutes (without the intermission) to nearly four hours (hideously long even though it was quite good).
Like mindelei says, plays often incorporate monologues, but a one-player show or play is, in essence, one big monologue. Currently there are quite a few in regional production--often someone bringing an historical person back to life (i.e., Mark Twain) or a single person playing a wide variety of characters.
Hope this is of some help.
Maryn