Here's a table that claims to give time equivalencies for cooking at different pressures. (This is from a website dedicated to pressure cooking information, so it's pretty standard.)
It doesn't go down to 7 psi, declaring 8 psi to be the lowest acceptable pressure, but the time increase for 8 psi is listed as 47%. Therefore, I'd try increasing the times by 50% or a little more than the recipe states.
Keep in mind that although this chart looks precise, it's really not. Pressure cooking at lower temperatures will significantly change extraction rates for various flavor components in you stock, for example. Some things will still cook out at roughly the same rate, while others will take a lot longer at the lower temperature. I'd guess that increasing cooking times by about 50% is a good place to start, but you may find that's too little or too much, or even that the result in some recipes just isn't right. (Sometimes boiling for too long alters flavors or textures in undesirable ways, so the fast cook in a high-pressure pot may be okay, but a slower longer cook may end up with just a boiled "overcooked" result.)