In Ruby's application we put 'up to 6 months' as the duration.
There is no specific form for a sponsor who is not a relative, but there is space on the VAF1 to give your details, then you need to add as much supporting documentation as you can (as specified in the checklist, which is also part of the submission).
As a minimum, you will be required to supply an invitation letter, six months of bank statements to demonstrate your ability to provide financial support, and some evidence that suitable accomodation is available.
