How long it will take to fill up a small swimming pool will depend on the size of the small swimming pool.
For a small kid sized plastic swimming or wading pool it should fill up within 20 to 30 minutes or less and for a larger inflatable pool it may take an hour to fill up.
Then for the larger swimming pools it can take around 3 to 8 hours and sometimes as long as 10 hours depending on how many gallons the pool is.
You can figure this out by timing how long it takes to fill a 5 gallon bucket of water and then times that time by how many gallons your swimming pool is.
For a 5,000 gallons swimming pool it can take from 9 hours to 15 hours depending on how your water pressure is and the size of hose you're using to fill the swimming pool up.