Tires

I noticed that my rear right tire was a bit flat--on the tire it says max. 51 psi. Should I fill it up to 51? or how high should I fill it up to?

I just got 4 brand new tires from Sears because the old tire from the rear right was deflating. Now it seems that this one is getting deflated too. Any idea why??
How many months should I go until I fill up the tires again with air??

The 51 psi is maximum tire inflation based on load. I would not put that much air in. IMO a good tire pressure is around 30 - 32 psi cold. You should check tire pressure frequently like minimum of at least monthly.

If the same wheel is deflating with a new tire then you may have a bad rim or an air stem filler valve if it was not replaced when the tires were done.