In natural areas, like where we go camping in a state park nature preserve in FL, pine needles are the main source of 'natural mulch.' There are tons of pine trees, some oaks, and various palms, all surrounded with tons of pine needles. A natural, native situation.
If used alone, they take a long time to degrade because they are so 'fluffy' and dry so quickly. For the first year or 2, until there develops a layer of much smaller, mostly and partially degraded bits that fit much more closely together, pine needles don't offer much in the way of moisture moderation and/or weed suppression. Stepping all over them after laying them would help flatten them somewhat, helping them to stay more moist and degrade more quickly.
If it's possible to add some leaves to the pine needles, the results would probably much more enjoyable/successful, more quickly. Organic matter degrades much more quickly when moist, and adding leaves would help with that (as well as balance the acidity, depending on the type of leaves.) Adding grass from mower bag when possible would also help. (Mow before grass forms any seeds to do this.)
