question on duration
 

i'm back on the exercise bike. what is a good amount of calories to burn in a session? i'm currently burning just over 300 per workout on the bike (about 45mins at a reasonable pace) and i'm doing this twice a day at the moment. i'm a little to heavy to jog (my poor legs)

The number of calories you burn during a time period is tied to how intense the workout is. The intensity of a workout is strictly individual. I know that I couldn't get near the intensity others can do. I have learned over the years that I progress very slowly. I'm happy as long as I am progressing no matter how many months it takes to get to the next level. Also, perceived exertion is different for everyone, and different factors affect the intensity level people can hit.

300 calories over 45 minutes sounds good to me. That's a nice moderate pace at 6.66 calories per minute. Here is a general chart for reference:

Intensity Time Calories burned per minute
Low 60 minutes less than 3.5
Moderate 30-60 min 3.5-7
High 20-30 min more than 7

Sorry, the spacing on the chart didn't convert nicely.
Hope you can still understand it.