Mine is 2 hours except for one position I took on, it's for 60-90 minutes (they pay for 90 no matter what). I took that because it fit with my other PT job I have that day, I leave one and go straight to the other and I am already close by (15 minutes with bus and walking). I make $30 for that one day per week, whether they need more or not.
ALL of my PT positions are actually guaranteed hours, with a 2-3 hour minimum. I don't do early morning hours, because I don't like it and I hate to do split shifts like that. I also have a set schedule for the most part, there is no parents coming home at 1am if I needed to get up early the next morning. Occasionally I will stay late, either set up in advance or due to traffic (like today, MB was home 30 minutes late but called to let me know). They ASK me if I want to work more, and how late I can. They don't TELL me or come home late without notice.
I just wouldn't be able to work with either of those families if they are having you work extremely late like that (assuming that it's a weeknight) or asking you to come in for 40 minutes in the morning - last minute -when you normally don't even work. All parents I know will sometimes run into an early meeting that they suddenly need coverage that normally doesn't happen, but they usually know more than a few days in advance for these, or won't assume that you can do it.