How long after adding water conditioner can I put my Betta back?

Yes, tap water may contain chlorine or chloramine. Either can be toxic. Chlorine will dissipate if water is left out for 24-48 hours. This is called aging. Chloramines (used by many municipal water systems) will not dissipate and must be removed chemically with a water conditioner. StressCoat by API or NovAqua Plus by Kordon are both adequate conditioners. Look in the aquarium section of the local Petco or fish store for them.

Ideally, you want to wait at least 4-6 weeks for the tank to properly cycle regardless of size. Bettas are quite hardy and adaptable, so as long as water chemistry and temperature are good then you can probably add him now since you have everything set up. When testing water chemistry, aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Water temp should be between 78 F and 80 F. A small filter and submersible water heater should be used. You will still need to perform partial water changes to stabilize chemistry and/or to maintain water quality. Use only conditioned water that is temperature matched to prevent stress or shock.

Creek water would not be recommended as you would likely need to heat it and the risk of waterborne pathogens (even in fast moving water) would be greatly increased. Ordinary tap water can be "aged" by leaving it out in a wide mouthed container for 24-48 hours prior to use. A water conditioner would still be recommended to minimize stress after water changes.

When performing water changes it is recommended to change about 15-25 %of the water. His helps keep some of the good bacteria in the water as well as decreases dramatic temperature changes which can cause significant stress on the body. I suspect that it is the temp change that is hard in him, however if a dechlorinator was not used in the water then that could be problematic as well. If you feel that is the case please Add a dechlorinator. I hope he improves!

Confirm stable water chemistry and water temperature. Bettas will jump out of the tank of stressed or if water quality is poor. They can breathe air for short periods of time as long as they stay moist. He may recover in a bit.

As long as you maintain pristine water quality via filtration and regular partial water changes, he should heal up. Use only conditioned water that is temperature matched to prevent stress or shock. Bettas do best between 78 F and 80 F. Consider a product, such as Stress Coat, to promote healing. https://www.petco.com/shop/en/petcostore/product/api-stress-coat-aquarium-water-conditioner

The frequency of water changes depends on how many fish you have in the tank with your betta, how large the tank is (i.e. how many gallons it is), how often you are feeding (more frequent feedings often results in more wasted food and more poop, so it can dirty the tank more quickly), and if you have any live plants present. The amount of salt that is needed also depends on whether you are adding salt just for maintenance or if you are treating your fish for stress or disease. My advice would be to get all of this information and contact your local pet store or aquarium store as they can then help to guide you on these specifics. Alternatively, you can gather all of this information and I can also do a consultation with you as well, whichever is easiest/most convenient for you.