I explained earlier on in this thread how it worked for my strategy.
If you visit an area for the first time, dig sites will spawn withing the hour. When you leave that area, the situation there remains 'frozen' in time. When you visit such an area again between 2.30 and 5.00 am, you'll find the dig sites spawn again, thus giving you the opportunity to collect 'twice'. It is not an ongoing process. Once you've collected twice in this way from an area, you would have to spend a lot more time there for dig sites to spawn yet again - unless the area is cleared and you revisit during the above mentioned spawning hours. The actual 'spawning' time does not seem to exceed an hour. That's why you can prep at least two areas at once.