Try taking advantage of SQL jobs.

We did the same you are trying to do like this.

Set up a Job with say 2 steps.

Step 1 searches for that file, if it is found goto step 2, otherwise try again in 5 minutes.

Step 2 Do what ever you need to with the file.

There are some more advanced ways with rescheduling the job from within itself but sounds like the first approach should solve what you are trying to do.