When I started to commute several years ago on a 16 mile round trip started with 2-3 times per week slowly increasing it to 5 days per week. Now my commute is shorter but more hilly. I still ride 5 days a week and sometime even extend my rides to get more miles. And yes I often ride on weekends as well. Your stamina will increase over time. Just take it easy and build up slowly at your own time and your own pace.