Very seldom. I am fully patched and have all the DLCs, no mods currently. I play on a PC that runs Windows Home Premium 64 with a good video card (NVidia GTX series) with drivers that are fully updated.
In the main game and most of the DLCs except OWB, I have had only a very rare crash--I can't even recall exact circumstances, as it was the kind of thing that happened once and on reload never happened again--no different from most other games I've played at some point.
In OWB, I had the aforementioned crash issue where sometimes when fast traveling, I'd get a crash to desktop. This happened several times, and always when fast traveling. Fortunately, it is a small enough world space that usually I just "slow traveled" to avoid it, and if I wanted to fast travel, saved my game beforehand just in case.
If you are running on a PC and you are getting lots of crashes, I'd suggest making sure you are 1) fully patched and 2) that your drivers are fully updated. If you're running on a console... *shrug* sorry, don't know what you can do.