safety kleen is a ripoff. trust me. also, their chems are hard on squeegees. read your terms of service with them, if they spill your stuff anywhere...you are responsible for the cleanup costs and liable for any lawsuits.
i get longer than that out of a thing of chems, you can pour the sludge out of the bottom and let the solids settle and skim the good stuff from the top. the chems never really go bad, just keep filling it up and once a year or once every 18 months I would deep clean it, pop the top, pour off the good stuff and let the rest settle in a 5 gallon bucket and skim the good off that to go back in.
you can get an oil filter relocation kit from Jegs and put a huge oil filter on it, that will extend the amount of time between cleanouts. without one i would imagine you would have to clean it out every couple months.