The conventional rule of thumb is to restring as often per year as you play per week, but no less often than twice per year. It won't hurt your racket to restring less often, but your strings might lose their responsiveness.
2. What's the best tension?
Generally, tighter strings offer more control, looser strings more comfort. Looser strings also seem to have more power because they tend to hit farther, probably due to their prolonging contact with the shuttlecock/ball while the racket moves upward with the stroke. String tension has a profound effect on the way a racket performs and feels. I've seen lots of players hate a racket strung at one tension, then love an identical frame strung differently. (This is a good point to keep in mind when trying rackets you're considering buying.) There's no single best tension, and the pros offer little guidance, with a huge range in their preferred tensions and no apparent correlation to style of play.