For releasing the lower back region i prefer to use a tennis ball over the biofoam roller. I am able to pinpoint the areas the need releasing a little better.
I would hold on any one area for around 60 sec but sometimes i hold a little longer till i feel the release or I would come back to the point to further release. You can perform SMR daily, and in some cases several time a day.
If you are having problem with your back then you may want to consider rolling the lats and piriformis also.