How often I file usually depends on what I'm cutting. If I'm cutting soft maple street trees, I may go a couple of days between filings as the saw stays sharp.
Once I put a chain on I never take it off until it's worn completely out..
I can file my chain faster than I can swap it out. . . .I have learned is not to wait till it is butter knife dull, . . . and a touch up only takes about 1/2 a cup of coffee.
...I prefer to sharpen at home, with good lighting...
If you are worried about getting the correct angles; a lot of chain manufactured by stihl has score marks on both top and sides of the cutter for reference.