40 minutes seems like a lot. We had our sensor away from the pump during bath on the first night and it has lost sensor within 10 minutes. I'd keep it on the counter during the bath, we started that and haven't had an issue. Put in the bedroom and we have an issue.
I think it's really trial and error and see what works for you. What worked for us is turning off high alarms while we are getting used to it and leaving low alarm at 65. The opposite or a modification of it may work for you. Just put the baby monitor on loud and you can't miss it. Although I am sleep deprived from constantly thinking I hear it and don't, I'm sure that will go away eventually.
Good luck! We just started ours last week, it's invaluable.
Logan 6 years old Dx 7/25/08.
pumping novolog 10/21/08 with ping pump and G4