If a project should take about 4 weeks to complete. Is there a reasonable time period of no contact before the contractor is considered to have walked off the job?
There is no reasonable time. The contractor should be in contact with you advising you of his schedule. It all depends on how much you've paid him up front. If he doesn't owe you anything then, after the 4 weeks is up, I'd get someone else to finish the job.
Search the TrustedPros directory and discover the best contractors in your area.
Find your home service pro