How long after stopping a regular weight lifting routine would you start to lose strength?
A general estimate is 10–15% strength loss after 6 months of inactivity. I know people who have talked themselves into being much weaker after just 2 weeks. I've also renegotiated this ‘mindset' weakness with a few, telling one, in particular, that he was, if anything, likely to be a little stronger for taking a break. He took this new ‘knowledge' and lifted 5 kg more than he ever had before. So he had just failed 110 kg bench press, his previous best, and was convinced he was ‘much weaker'. I assured him that this was just in his mind and that, on average, he should up to 5% stronger, after a two week break. He made the 110, then 112.5, then 115 kg (242lb, 247.5 lb and 253lb). The last two, being personal bests for him at the time. He was intrigued by this new knowledge and curious why he'd never heard it before. I explained to him, that he couldn't be blamed for that because I'd only made it up 5 minutes ago.
Bottom line: Don't trust anything I say, or possibly, don't fabricate an ‘I'm weak' story. The thing about this is it's usually true. There was no reason to believe he'd lift more but there was also no reason he should have lost noticeable strength. By re-casting his time away from training as a break, coupled with my ‘new knowledge' he overcame the real hurdle, which was in his head. Now all that said, feel free just to dial it back a bit if you have had a break and get the first few sessions with relative ease, to build confidence and momentum, then smash your previous PB.