Very rarely.
I can count on one hand the number of times I have had a crash or freeze since I bought the game on release. I have my issues with the game, but crashing isn't one of the them. I am playing a pretty heavily modded game.
If you are experiencing freezes, then there is only really one place to look for the culprits - mods. Playing mod roulette is very likely to result in a crash/freeze. It's very important that when using mods that you are organised.