Depends on the level of fitness, but I think it should be done in around 3 hours(70 km, so you have to do around 23,4 km in one hour).
Professionals do the 90 km in around 3 hours or less, depends on the wind around the lake,the traffic(esp. in zurich bellevue and rapperswil) and the bike(if you got a bike what is less then 8 kg) and the fitness.
I did it today with my MTB (with small breaks took me 3 hours),or you can start with a trip from zurich to Wädenswil (it is around 20 km one way)