Way too close if you ask me. Sat. night at TLV is the busiest travel time, since LY shuts down over shabbat, and will be ramping up all its post shabbat evening/night flights. You need to get yourself to TLV 2 hours minimum, 2.5-3 hours optimally, for adequate time for security, etc. there.

If you're only leaving jerusalem at 17:50, I'm guessing you're only going to arrive at TLV slightly before an hour prior to departure. You'll also want to check the minimum check in time for TLV, which is usually earlier on most airlines than other airports.