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.
Over training is incredibly hard to get to, elite athletes train every day, heavy without signs of overtraining(1)
Also Schoenfeld conducted a study where he compared the effects of different training volumes on the average gym rats.
Even though people in the experiment were doing 45 sets, weekly for their legs they have experienced 0 signs of overtraining while having bigger muscle growth effects than the lower volume groups.