It is really important to carefully read what you are playing. Usually when mistakes are made students dont even realize it. Take the time to double check yourself on this, or, if you can, have a practice session with a classmate. Subdividing can solve many problems when you work slowly. When you are figuring out rhythms, divide the beat.
Music is very mathematical and so are solutions to rhythm problems.
Reading four quarter notes in one measure may not seem to need subdivision, until the surprise syncopation happens in the next measure or bow changes and slurring are awkward the first time you play a passage. My suggestion for students is three hours per week minimum. Usually, as you progress with your study, you become more efficient with your time, but that minimum three hours is for everyday reinforcement. Playing an instrument is a very enjoyable experience and can be very soothing. Sometimes it seems that you have to sacrifice other things in order to fit in time for practice, but I think you will find, as you progress as a musician, not practicing could be a harmful sacrifice to make.