It is very hard to say. A LOT will depend on geography, how your TJ is sitting, and what is coming up.
When I laid mine over last year, I was at about 35 degrees and it felt okay but leaning good. I then proceeded forward and my low side front wheel dropped into a hole and it quickly went to 90 degrees. You can see where my tire fell 'down' and slipped.
This is the result.
There are other times I've been on a smooth surface at 40 degrees and been fine. Soil conditions, etc all play a role.
I'd say if I HAD to put a number on it, say below 35-40 degrees and you're okay, for the most part, but again, terrain is the big factor.