Hi Sarainia!
The choice is up to you. Check out some of the games from last year: https://itch.io/jam/teenytiny-dragonruby-minigamejam-2020/entries
Some people chose to use a countdown timer and the player has to try to score as many points as possible in the time. Others chose to give tasks that take approximately 20 seconds to complete. It's up to you how you want to interpret the 20 second rule and as long as your game is within the spirit of the jam, it's all good.
The jam runs for 3 weeks because we want to give everyone a chance to make a game without any stress. That includes beginners and people with busy lives. If you are able to finish your game with plenty of time left over, feel free to make more games :D There is no limit on the number of games you can submit.
The link you found is the download page from last year. This year the plan is to make the engine free here on itch.io for a limited time (check back around September 24).