How often you need to recharge depends on three factors:
1) The hardness of your water, measured in grains per gallon (test, using the kit mentioned)
2) the softening capacity of your unit ( e.g. 30,000 grains)
3) The number of gallons your softener will treat, based on capacity and water harness it is treating.
To devise the number of gallons the softener will treat, per recharge, siply divide the capacity figure by the hardness figure.
For example: If your softener is rated at 30,000 grain and your tested average water hardness is 10 grains per gallon:
30,000 divide by 10 equals (=) 3000 (gallons)
The hard part, (no pun intended) is determining how many gallons you might be consuming, so you can estimate the longevity of a charge cycle. Most users just find it easier to test the output water periodically to find out approximately how long the softener charge cycle lasts based on their unique cleaning style. Once you have tested through a couple of cycles, your conclusions are going to be accurate enough. Some find, if their water is exceptionally hard, that testing is almost unnecessary because they simply notice when the charge is depleted (the cleaning efficiency drops). Once you gauge your unit, you simply charge it before it's scheduled to deplete.
As for "changing" your resin, it really should not be necessary unless it becomes highly contaminated or fouled (which a working strainer should prevent). Most people find the only reason to add resin is when and if they lose resin due to spill or leakage.