>> How do you know when to harvest,
Harvest continuously. Definitely you should keep the plants well-picked so that more pods will form. If you like small amounts of very tender gourmet pea pods, pick them small.
Most people say to pick each pod before it gets "too large", on the theory that they are more tender and sweeter when less than full size. I don't know, I also like them fairly large: big crunchy pods that are still sweet, with the peas big enough to make the pods a little "bumpy".
>> and when the vines are done for the season?
Eventually my vines turn yellow or brown and/or get white stuff on them (?mildew?). I'm sure that is "past their prime".
My summers are so mild that I don't think they are responding to heat stress, just old age. I assume that the climate in New Mexico will dictate the end of Spring peas when you get a few hot days. Do you have a long fall, or does frost follow summer heat quickly? maybe a Fall crop of snow peas will give you a longer window.
I've never had enough snap pea or snow pea plants that I could freeze any pods or even save a mess for diner. I always ate them as I picked them, or brought a few handfuls to work to eat fresh.
