As far as I know, a chemical pregnancy only lasts about until you expect your period. It's not very long at all. That's why they suggest you don't test until after you've missed your period, so that you're not disappointed by a chemical pregnancy.

A chemical pregnancy can last for several months with the would-be mother believing she's actually pregnant. Most will finish within the first month but it is not uncommon to have them last two or even three months.