I've not missed a day for approximately 6+ months, and my prescription ran out so I don't think I'll be able to take it again until tomorrow (will have missed essentially 3 days).
How damaging is that in terms of my finasteride use?
not noticeable in the least, I took propecia for 11 months then dropped it, and it took my hair 2 months before I saw the difference, which was substantial but I digress. I've heard a couple people who are pretty educated in the area say it takes about 2 weeks minimum for your hair to begin to be effected on a very basic level, so no worries a few days here and there don't matter.