Although Sage seems to have trouble in the US midmarket around vocalizing their policies, I do see and expect that they will soon globally adopt an N-1 strategy (similar to what they've just published for X3 - see attached pdf) .
Support for their releases will be current and one prior version. This is in line with what they are publishing now for Sage 100 (product updates only for current and one prior release).
I believe it's worth guiding customers to this reality. If they are paying maintenance they should be upgrading annually since in order to obtain all the value of maintenance you must be on an N-1 release.
Source: X3 changes to support (which I'm presuming will be globally followed in some way).