It varies from man to man - while most do, not everyone loses their erection every time they ejaculate, and the time to get another erection varies enormously. Erections involve parts of your nervous system that are not under conscious control (if this were not the case, you could always have one when you wanted and never have one when you did not...)
You don't need an erection for your or your partners to have a good sexual time, and regarding coming sooner than you'd like, yes this can be avoided with practice. See, for example, whichever is cheapest of Men and Sex or The New Male Sexuality by Bernie Zilbergeld.