I used to be a competitive swimmer, so I did LOTS of laps for exercise
Really there are no set rules for swimming - the more often, the better, though I would suggest at least 2 rest days per week.
For a start, I'd recommend 3 sessions a week of roughly 45-60 minutes. The number of laps you can do in this time really depends on how good your technique/stamina is, so if you want to improve I would suggest you count your laps in your initial session, then gradually try and do more laps per session. For example, if you manage 50 laps in 60 minutes on the first attempt, try for 52 laps on the next session, then 54 etc. Once your stamina improves (and it will fairly quickly if you keep this up), you'll soon be doing 80+ laps!
If you want to gain a reasonable amount of muscle, you will need to supplement the swimming with some weight work - swimming will build some muscle, and tone in nicely, but it is mainly an aerobic exercise, so for muscle building you need some weight work too. One or two weight sessions a week should suffice if you complement it with your swimming.