Usually they germinate within 2 days, although Ive had one or two which havent so ive left them in a glass of water out of curiosity (easier than a paper towel for long periods coz it obvs doesnt dry up) and some have taken as long as a week and a half to germinate (no noteable difference to the success of the resulting plants either). Ive heard reports of some taking over a week as standard, never come across anything like that though.