The max would be 100%.
It is called B100, but you are partially correct in that most often it is found in 'fleet' and 'public' pumps at B20 or 20% biodiesel. (can be both virgin or recycled oils)
The reasons are multi-fold. 1) It is still a newer fuel that has yet to have been approved in all diesels in the US in higher percentage blends. (warranty concerns) 2) It is generally higher priced than diesel but with high petroleum and tax incentives to 'blenders' is much closer now. 3) The initial introduction for fleet useage included a tax credit to the end user (schools, cities, etc) of up to 20 cent per gallon: 1 cent for every percentage used ... max 20 cents. Hence the B20 blend) 4) It has a offbeat 'homebrew' component, multiple upstart small processors and no independent quality checking mechanism ... if you built engines would you warranty for 100% until it was fully tested and quality was standardized and checked coast to coast? 5) There is a cold weather problem as B100 gells at a higher temperature than D2. Additive packages help, but resellers are businessmen and find blending D2 (wintertreated) solve the biodiesel gelling problem. 6) I mention business ... biodiesel volumes are not large enough yet for many to justify using a pump for multiple grades ... many just select one blend if they are planning to offer biodiesel at all. (this goes for everything up the distribution chain. Storage, shipping, pipelines, etc. Demand OR Supplies are not there yet making it another product to handle.)
This is still a new industry with a few hurdles, but the use, growth and support is rocketing its development and acceptance at an exponential pace. There are bound to be a few hiccups as well as battles with the oil industry reluctant to give up any market share or dominance over energy for vehicles. It will take time but is enjoyable to be in at the grassroots. (I'm guessing many of you feel the same way with Hybrids? You know the car you have today will be 'topped' by each year as technology advances, but it is something you can do in supporting efficiency and better emissions today so you back your convictions with action.)