For individual seams I have a "3 times and your out" rule. If I've had to rip out the same seam twice, the third one stays even if not perfect. If the 3rd try is really unacceptable I'll put it aside and start another block. Sometimes at the end I'll have enough scraps leftover to replace that bad unfinished block.
There are days when nothing goes right. If I've had to rip out 3 different seams within 15 minutes I'll quit for awhile.