It's exponential.
Lets pretend they can only make two eggs. Egg morphing takes about 3 hours tops. So if you nab two hosts at the same time, within 3 hours you have to more eggs. If those have hosts about 16 hours (give or take) later you have two more Aliens. They morph two eggs each, etc etc. So if they can only make two eggs each and if they only live 24 hours, you'll still have a constantly replenishing and growing supply of Aliens, even with those restricted conditions.
Queens on the other hand take 2-3 days to gestate then another 3 days or so to start laying.