How long after sending a letter of intent to you have to wait before filing a mechanicsblien with the clerk?

There is no waiting period. You just have to send the intent to lien notice prior to filing the lien affidavit. Once the lien affidavit is filed, you have to provide the owner and the general contractor with a copy of the lien affidavit within five days.

You don't have to wait at all. I have filed a mechanic's lien right after mailing out a notice of non-payment.

Retain a construction attorney to evaluate your legal position and to provide advice.

Good luck.