Saru lake trek is definitely not a difficult one if you're properly equipped! So whatever time the locals say, just double that time. You should start around 7 in the morning so that you can reach at the earliest in the evening since it really starts to get chilly and the winds just push you off the track! Wear a good jacket and carry a stick since you might have to cross steep glacial layers to reach the lake. Also, do hire yourselves guide for the trek because you might get really lost as has happened with many trekkers in the past. Carry water bottles and few things to eat and if the localites say that it might get really tricky or it's not advisable to go for the trek then do listen to them!! It's good to be adventurous but not at the cost of your lives.
One can go for this trek solo also but ofcourse do hire local guides/porters alongwith you according to your needs. It should be done in 10,000 from the stay, buses, meals etc. everything.
It all depends on how long you want the trek to be, so there is no defined route. Usually the travel packages offer the following route:
Best time to visit
Best time to visit is between May-October. There might be snow sometimes since we visited in June and we got to witness snow!