Release windows – entitle or constrain?

I’m currently doing some work planning releases for the next 30 months or so based on some standardised models etc (more on this in another post).

I’m scheduling 2 main types of release: functional releases in the main business systems (running SAP as it happens) and maintenance / patching windows.

The organisation has a real appetite for business functional change, and is less keen on regular maintenance. However we have to comply with corporate global policy on WINTEL and linux patching as well as app patching so we have some political support for this vital work.

Building this into the schedule therefore leads to a curious question – when we plan releases far into the future, do we do it to constrain their activity and impact on other releases, or entitle them to have that reserved slot…or both? What do you think – for your varying release streams, do you schedule activity more to constrain or to entitle?