Silicone caulk does not dry. It cures. That means a curing agent in the silicone must be active and working when it is applied. Different brands, different grades, quality of caulk, and different factors such as humidity, location, application, heat, and exposure to light can all affect the rate at which silicone caulks will cure.
Different Silicone Caulks
Not all silicone caulks are created equal. Some dry faster, some slower. Depending upon the formula and the type of silicone you may only have to wait 24 hours before you can use the tub, sink, or other item you caulked. There are also fast-drying caulks you can use that will still have a long cure time but will require a shorter wait before use. Drying and curing times for individual caulks will be written on the packaging label.
Make sure you have a true, 100 percent silicone caulk and not a blend of silicone and acrylic or an elastomeric latex sealants, such as DAP Dynaflex and Polyseamseal. Elastomeric sealants aren't true silicone caulks, although they are sometimes mistaken for such.
Things That Will Delay Drying Time
Silicone caulks have an expiration date. If the caulk is out of date, it may never cure. If in doubt, test a bit on a nonporus surface to see if the caulk gets rubbery and pliable after 24 hours. Most silicone caulks will dry to the touch within 30 minutes to an hour; this does not mean the silicone has set, but if this much doesn't go according to instructions, this is your first sign that something might be wrong. Expect an average of 24 hours for most silicone to cure, so if it has not cured by then, you should consider buying a new tube of caulk for your project.
Humidity levels can actually also affect your cure time. In a moderately humid climate, the 24-hour curing will hold fairly true, but if the air in your climate is drier, it could take up to two or three days for your caulk to fully cure. This can also be affected by the time of year. Commonly, cold winter air is very dry, so if your caulking project happens to fall in the colder part of the year, your curing will usually take longer.
Accelerating Dry Time
It is generally recommended to simply follow instructions on the silicone caulk packaging rather than trying to speed up curing time. As mentioned previously, there are some faster drying formulas of caulking available that will dry for use well before a full cure.
Using heat is not a good idea. Heat softens silicone caulk and can disturb adhesion. It is actually sometimes used to aid in caulk removal, rather than to speed up cure time.