if they are fungus gnats, then their life circle is only two weeks (14 days), but each female adult carries 200 to 300 eggs... so they do spread out fast if you don't treat them right away. Once indeed, you have identified that it is fungus gnats that you are having... which I have a feeling that you do, then go to Sheridan garden at 4077 Hwy 7, Unionville, tell them that you have a fungus gnat problem, they should carry an insect spray which is specifically for that. $10 I think for 250ml... Note: here is my tip. before you begin your spray, you can only use half of it and mix with RO water, thus save the other half for the 2nd round... only spray it directly to your soil (the spray doesn't harm the adult (flying ones), but will wipe out all the larvaes, and with the 2nd round after 14 days for the insurance. If they fungus gnats, then it's a sign for two indications: 1. your soil is ideal for house plants (good) 2. you are over watering (something to watch out for)
good luck.