I'm going to say that it's about 125 miles or so.
I think it's 150 to the troy locks..
So if you can average 4.5 knots for 8 hours a day..it will take you about 3.5 days...very rough guesstamating...
The river is tidal the whole way so you will have to take that into consideration. Wait for the tide....Around NYC the current can run 3-4 knots..or more..you can literally stand still if you can't make good 5 knots..
I don't know how strong it is 100 miles north.... I know I've seen ice flowing up river in Hyde Park. You'd have to look at an eldridge.
I've give myself 4 to 5 days..minimum.....and more if you want to stop and see some sights. Depending on when you leave you might catch the clearwater festival...in croton