Can a free Heroku support 500 users/day who spend around 3/4 minutes on the site?

Yes, with a couple caveats.

500 users/day is ~20 users/hr, which isn't much traffic at all. A single dyno is more than capable of serving this kind of traffic. But the real bottleneck is the database. If users are hammering your database during their time on the site, this will bring everything to a grinding halt.

The other caveat is free dynos on heroku are limited to ~500 hrs/mo. Given a full month of running is ~720 hrs, obviously your dyno can't be running all the time. This is usually fine since there are hours of the day where users won't be on your site but know that you may run out of hours before the end of the month. The next tier dyno is $7/mo, which is pretty cheap insurance to make sure users can access your site.