Around 10 minutes, you get a comment. Over 20 minutes I'll likely be pretty grumpy about it. Over 30 I'll likely just go about my day without them. There are exceptions though - like if they keep me posted and have a good reason - then cool, I'll understand. But if theres a habit of not respecting my time I'm not going to tolerate it for long (yeah - I'd totally stop being friends with someone if it became a habit).