I don't think less than an hour is enough time to really get to know someone and know if you would consider a second date or not... So, maybe 1 hour or more?
Personally, my first date with my boyfriend lasted 6 hours, we did lots of activities, and got to know each other more than just the awkward surface-level introductions... So that time spent together made me 100% sure I wanted a second date, (otherwise I would have cut it short). Typically, I think that 6hrs is a bit long for a first date, but if it was just 1.5 to 2hrs, I think that would suffice. The 6hr long date signified that it was a good and enjoyable date.