We know that the Imperial Guard is the meat shield of the Imperium and the main force. But how often are the chapters called, them being the elite of the Imperium. By deployment I mean from space marine squads to terminators and even dreadnoughts, knights, titans.
The books detail everything from a single marine being deployed to several companies to the whole chapters.
Single marine - Iron snakes book
a few squads - All Ragnar books and countles others
A few companies - Armageddon war event that happened a few years back had multiple chapters send everything they had to stem the tide.
the entire chapter - tyranid invasion of a Chapter home world would qualify involving the entire chapter and all its allies. ( Ultramar was attacked in this way and almost destroyed )
It depends upon the threat. It also depends if it is a Chapter world that they are sworn to protect. It also depends if they have men to spare.
In the grim future there is only war. In war there is no rest for the Space Marines. They are constantly going from a hot zone to a hot zone. The only time they are "resting" is when the company numbers and materiel drop so low that they HAVE to return to their staging point or home world to resupply and rearm.
To answer your question they are always out there. Space Marines are honour bound to help if they receive an astropatic call for help. Problem is that they receive so many all the time, they have to pick and prioritise.
An Allied world of the chapter would get priority over a small colony. A forge world would get priority over a hive world. (men you can replace pretty fast, machines and factories not so much )