One hour and forty-five minutes
From 4:30 to 5:00 it's 30 minutes, from 5:00 to 6:00 it's one hour (we are at 1 hour and 30 minutes now), and from 6:00 to 6:15, it's 15 minutes.
In total, it's 30 minutes+1 hour+15 minutes which can be rearranged to say
1 hour+30 minutes+15 minutes that you can simplify to 1 hour+45 minutes or
1 hour and 45 minutes