Depends on the area of the forum & the rules in place there.
e.g. the market allows a bump after 24 hours from the original post. Other areas don't like bumps full stop - the idea being that if your thread isn't interesting to other people already bumping it isn't going to help.
Looking at what you said (10 + 15 hours), bumping like that should be ok in the market - but hanz0's incredibly strict about his rules so if it looks like you've tried to get around them he'll react to it.
Take note that in the market forums you can't bump even after 24 hours or so unless you have a VALID reason for bumping such as changing the details of the sale, adding items, etc.