Time how long a fetch takes, add at least a few seconds to that, and then wait at least that long before starting the next fetch. That should do a pretty good job of preventing server overload for batches of requests that only get run rarely.

Note that I mean for you to time each fetch. If the server gets bogged down, then your script should immediately notice that the previous fetch took longer and automatically compensate by waiting longer before trying the next fetch.

Thanks.