One per 2 months is the plan (which would mean the nearest one being availible on the patreon somewhere in may).
Viewing post in Soles of Divinity [Footdom RPG Demo] comments
If you get in literally today, the most you'll get is access to updates and polls to decide how the rest of the game will be shaped.
That being said, I plan to make early test build availible at the start of may for patreon volunteers, and actual update with fixes released about week after that. So in short, you'd get to play the 1st major update before the 2nd payment date would come.