The amount of miles that you can bike in 30 minutes depends on how fast you're riding your bicycle.
If you average a speed of around 12 miles per hour on your bike then you could bike a total distance of 6 miles in 30 minutes.
If you road your bike at 6 miles per hour then you would bike a total distance of 3 miles in 30 minutes.
At 12 miles per hour you would cover 12 miles in 1 hour.
At 6 mph you would ride your bicycle a distance of 12 miles in around 2 hours and maybe a bit more depending on if you stop for a break during that bike ride.
The slower you go the less miles you'll cover and the faster you ride your bike the more miles you'll cover.