IIRC, (and it's been awhile since I taught)... The endorsements do not expire. However, if I was the instructor that was sending a student up for a checkride (and the one signing the 8710), I would definately be sure that the student met the various experience and knowledge requirements, and that I was the endorsing instructor for all of the required endorsements.
(Does that make sense? Its still a bit early...)
Mainly just to make sure that my student is good to go and doesn't blow my record. For example, if I received a student from another instructor that was already endorsed (recently), but for some reason this instructor was unable to fill out the paperwork for the checkride (say he got hired somewhere and left short notice). I would tell the student that I would endorse them, but they first had to show me they were ready, by covering all of the materials in the FARs(briefly, either in the plane or on the ground, and in a minimum of 3 flight hours) . I would then re-endorse the student and sign the paperwork for the checkride.