I'm at one of the legacy big 3.
You can drop trips as much as you want. Our old contract said you must fly 50% of the following months hours but I'm not sure it is in the new one.
Here are some general basics:
You have to be senior enough to get halfway decent trips that others will pick up pilot to pilot. If staffing permits, you may get the ability from the company to drop a trip into the open schedule, but since they have been short of pilots it rarely happens.
This month and last, I had trips that were in demand for other pilots. I could have dropped every one without pay and had at least a month off. The nagging at home to pay the bills would have become unbearable from the Mrs.
You can also back trips up and leave the back of this month free and the beginning of next free for a long break.
There is flexibility, but one needs the right trips and seniority.
On reserve as CA or FO? Forget about it.