How much time should you study for exams?

It really depends on how much time you have left and how well you already know the material. If it's still four weeks away, I'd say it's better to study regularly instead of panicking at the last minute. Like 1-2 hours daily is often better than doing 6 hours straight once a week, because your brain retains stuff better when you spread it out. But more intense sessions on fewer days can also work if you find you're more focused that way - some people just need longer, uninterrupted blocks to get into flow.

What I'd check instead: how complicated is the material really? Bio can be all over the place, from memorizing random terms to understanding complex processes. If you have to memorize a lot, you need more time for review. If it's more about understanding, it often helps to explain the material to yourself or work through practice problems instead of just reading. Maybe start 4-5 weeks beforehand, 1.5 hours a day five days a week, and see how it goes - if you realize you already know a lot, you can easily cut back, if not, you just ramp it up. And honestly important: check for yourself whether what you're learning actually sticks, instead of just mindlessly cramming through the material.