The simplest way is to use ELISA. As for mice you need measurements in a small volume I would recommend Mercodia Ultrasensitive Mouse Insulin ELISA (Cat Nr 10-1249-01) which can deal with 10 microliters.
I have done a manual in-house competitive binding insulin I-125 assay (the best Gold standard assay I have ever used for Ins analysis) for quick results - takes about 20 minutes. We used this for patients undergoing surgery for insulinoma.
I have also used the LINCO elisa assay. You can get a result reasonable quickly esp if you are only after a low or high number and don't want to use the whole std curve.
I am about to test out a new kit from Alpco - the Stellux platform - a chemiluminescent EIA - let you know how it comes out. It has a 2 hour primary incubation and then you wash, add the chemiluminescent reagent and read.
I have used Millipore EIA and RIA, Crystal Chemical EIA and Mercodia all with good results.
If you are doing your research in China, you might have difficulty. We had a colleague who returned to China and found that she was unable to measure insulin perhaps due to poor handling of kits in transit and her workplace did nt have a way for her to use RIA. She ended up sending me the samples.