A toilet tank should refill within about three minutes after the toilet is flushed. If it takes longer, the problem is often that the shut-off valve is not completely open or a possible malfunction exists with the tank's fill valve, which regulates the amount of water to fill the tank.
Twist the shut-off valve further counter-clockwise to increase the water pressure of water entering the toilet tank. Look on the wall behind the toilet's left side to find the shut-off valve, usually located approximately 6 inches above the finished floor.
Remove the lid to the tank, and flush the toilet. Observe how the water refills the tank, whether it is flowing through the fill valve faster than before. If not proceed to step 3.
Twist the shut-off valve completely clockwise. This shuts off the water to the toilet.
Locate the fill valve on the left side inside the tank. Remove the top of the fill valve by twisting it off by hand or by removing the screws in the top with a screwdriver if necessary. Hold a small cup directly above the fill valve.
Twist the shut-off valve counter-clockwise, allowing water to flow through the fill valve. Block the upward-shooting water with the cup. The water flow will clean out any valve debris. Wait approximately five seconds, and twist the shut-off valve completely clockwise.
Reinstall the top to the fill valve, according to how you first removed it.
Twist the shut-off valve counter-clockwise, and watch how fast the water refills the tank. If the water is still slow to refill, the remedy is most likely a fill valve replacement. Twist the shut-off valve completely clockwise.
Flush the toilet and hold down the handle to remove as much water as possible. Soak up the remaining tank water with a towel. Disconnect the supply line beneath the tank with either your hand or an adjustable wrench. Remove the locknut on the bottom of the fill valve, underneath the tank, with an adjustable wrench.
Lift the fill valve from the tank. Wipe around the hole in the bottom of the tank, from which you removed the fill valve, using a towel. Install a replacement fill valve.
Install the locknut on the bottom of the replacement fill valve, tightening the locknut up against the bottom of the tank. Hand tighten the supply line to the bottom of the fill valve, and finish tightening it with an additional 1/4 turn, using the adjustable wrench.
Turn the shut-off valve counter-clockwise, and observe how fast the tank refills. Check for any leaks beneath the tank. Place the lid on the tank.