Have it analyzed early if you can. The older the sample the greater the uncertainties. Results don't age.
That said, keep it in the dark and keep the temperature cool & stable. Using a non-leachable container would also help. I have a couple samples in canning jars waiting for $8 kits.
Your highest risk from sitting is evaporation of fuel contamination, so it is a good idea to send relatively soon after draining. I send samples to my Lab on the 1st of the month and somewhere in the middle of the month. Anything that gets to me on the 2nd waits a couple of weeks.
For Engine tests I always send about 100 ml. and keep another 100ml as a counter-sample in case anything happens.