[JokerCPoC]

I found this on Hardware Acelerated and this is what You and I need to do:
If your old GPU heatsink is epoxied on, I suggest putting your graphics card in the freezer for a while, then using dental floss or another strong thread or wire to slowly cut through the epoxy - just ripping it off is likely to damage your gpu, and kill your card.

Now as to How long, I'd try overnight and Use a Freezer Bag or storage Bag (Not the Type with holes in the sides though) to protect the card from dirt and debris, Regardless If any exists or not that is, You can leave the bag open though. I hope this will help as I have to do this too when I get all the water cooling parts that I need.