Hello, the probabilities are no so high but they are still there, on average your chances of catching HIV would be around 1 in 2000, only if you partner was HIV+.
Testing is advised in your case, third and fourth gen Elisa tests are among the most common ones nowadays, according to CDC, any HIV test is conclusive if taken 12 weeks after an exposure, but you can take them before that, 3rd gen will give you an excellent indicator about your status around 6 - 8 weeks after exposure and 4th gen 4 - 6 weeks after exposure. A result given in those times is very unlike to change, but only a doctor, based on your risk level and his experience could advice you to skip the confirmatory test after 12 weeks post exposure.
Hope this helps.
Would my chances still be 1/2000 if it only lasted for 20 seconds?