To get a taste: 30 minutes
If you just want to try what boating feels like, half an hour is enough. It's cheap and, especially with small children, often just right before patience runs out.
The classic: 1 to 2 hours
For a relaxed loop across the lake, one to two hours is ideal. In that time you get around well, can take a break on the water and enjoy the calm without it getting tiring.
The whole day: day ticket
If you want to spend the day at the water, maybe add a picnic and head out again and again, a day ticket is the cheapest option. It is worth it as soon as you plan more than three or four hours.