Days since last strung: Literally the number of days the previous stringing lasted. If string breakage is not a problem for you, you might want to take advantage of the increased playability of thinner strings. If you are breaking strings frequently, you might want to look at thicker strings or strings designed for durability.
There are several rules of thumb for deciding how long to wait between stringings. One is that you should string your racquet as many times in a year as you play during a typical week. Using this guideline, if you played twice a week, you would string twice a year. Another rule of thumb is to restring after 40 hrs of play. If you find yourself wanting to smash your racquet to smithereens, that is also a good indicator that it is time to restring. You might also want to visit my shop and have the string plane stiffness checked. Low readings are a definitive indicator that it is time to restring. Historically, most customers find their racquets difficult to control once the string plane stiffness readings fall below 30 on the Beers ERT-700 or 48 on the Pacific Multitest Computer.