Can anyone give me information or direct me to a reference on maximum LC mobile phase life? We typically use buffered mobile phases with about 25% organic modifier. We would like to reduce the daily mp prep time by making up larger quantities at a time for extended use.

Use a one liter bottle.
Make sure it is amber.
It will be ok for ~ one week.

I don't know of a good reference. There have been some good discussions on the use of biocides in previous threads.

I think that greater than 20% organic of any common HPLC organic is surely biostatic and you could have extended expiration dates. I read someplace that 2% ACN, 10%MeOH, and 7%IPA were biostatic. We give all orgainic containing mobile phases a two week expiration and haven't had any problems. When doing standard and sample stability I also set aside some mobile phase and do one run with two week old mobile phase and make sure everything looks ok.

Amber bottles are useful for storing non-biostatic solutions like buffers without organic.

I doubt that there is a universal answer to this. 25% organic is likely to be biostatic, but for long term storage I would pass final buffer soln. through 0.2um filters, fill the amber bottles to the neck,or N2 flush the void, seal carefully, and refrigerate at 3-4C

Why don't you make some up and test its life? There isn't one answer to your question, but I never encountered a mobile pahse that wasn't stable for longer than a day. some of our runs were more than a day!