My regular workout consists of about 3/4 sets of 90 pushups and furthermore some situps. I am 19 years old. Would it be bad to do this workout (almost) every day of the week? And would doing it as often as possible give me maximal results or would I get better results if I took days of rest in between? If so, how many days a week should I work out?
I would recommend taking at least one day off for some light cardio/stretching/yoga per week.
That said, you should not be working the same two muscle groups on consecutive days. Your body needs time to repair the broken down muscles and cannot adequately do so in ~24 hours. I would recommend working in exercises that target different muscle groups for optimal repair and gains.