Vacant Property in Hot Market - How soon & often should I follow up?

[Account Closed]

I am interested in a vacant property, but have yet to get a response from the out of state owner.  I sent him a letter last month and followed up with a phone call (voicemail) a little over a week later. 

I am curious how often/soon you follow up on a property.  It is a really hot market and well done rehabs are bringing in some high prices. 

This 2BR, 1BA was purchased for $80k in January and sold for $245 last month.

I don't want to be a nuisance but I also want to be fresh in his mind when he decides it is time to sell.