How often should I calibrate the bed?

Not very often, as long as you don't manually move the bed or press down on it with any force.
You should only need to calibrate if your first layer isn't sticking properly.
The first layer should look "squished" (or flatter) and slightly wider than the following layers.
I run a test print after every half dozen or so prints to make sure the bed is level.
The test print is only one layer high as I'm only interested in the first layer and it covers about 3/4 of the bed to check how well the first layer sticks at varoius points on the bed.

Here you go.
Like I say, it doesn't quite cover the whole bed. I only designed it to cover the part of the bed I use the most, but you could simply position it on the bed wherever you like before slicing the STL.
Print it at 0.3mm layer height.