If you have no mobility problems, then the assent may take you around 15 minutes but you will want to stop a couple of times to take photos. Once at the top it isn't as crowded as the middle stone step area, which gets very congested and can slow you down. Take water with you as on a hot day you will need it. Enjoy.
How do you get up to the Acropolis?
The best way to get to the Acropolis is to take the metro to the Acropoli station. From here, you can walk for 10 minutes to the site.