For competitive markets 3-4 weeks is good, for less competitive tier 2-3 markets, you can get away with less mailing frequency 4-6 weeks. 

The two components that drive this are marketing; media, message, and frequency as well as determining the fail safe number.

I would suggest asking other users to see if they could share letters/postcards that worked for themYou must also plan the frequency of your campaign.

Frequency is MOST important.

So the more times a seller understands that they have a choice the better.If you want to increase your success rate you'll need to increase both your frequency and presentation.If a prospect receives 4 completely different pieces of mail from you with different messages and contact info within a short period of time and 6 pieces from other investors the odds of generating a Lead hugely weigh in your favor.Also keep in mind the value of a touch.

"whole slew of variables" Wholesaling and marketing -depends on many aspects that @Jerry Puckett can answer, but FREQUENCY and BRANDING is important.

Keep in mind BBs is a frequency and reach marketing model.

TNT -TV $30.00 HGTV-TV $53.00 AMC -TV $24.00 BRVO-TV $40.00 OWN -TV $13.00 FS1 -TV $15.00 USA -TV $50.00 FXNC-TV $63.00 ESPN-TV $50.00 TRAV-TV $14.00 FX -TV $34.00 SYFY-TV $17.00 ID -TV $5.00 TVL -TV $15.00 OXYG-TV $9.00 FRFM-TV $29.00My spend is 15k a month and gets me a 49% reach and a 12 frequency. 

I do some code violations, unlawful detainers and probate marketing however the list size is small  I’m also spending 15,000 a month on TV in my market that gets me 49% reach and a 12 frequency PPC YouTube and Facebook I am enjoying as well. 

Michael Quarles has a number of posts about direct mail that you could review, especially about minimum frequency.