I see that you ask two different questions, about keeping PC offline in general and scheduled settings in particular.
In general, you can keep PC offline or switched off indefinitely. Of course, when it goes on/online and some important tasks, needed to keep OS in the healthy state, were missed, OS will try to do them ASAP, which can make PC semi- or almost completely non-responding for a few minutes, especially if you have missed important system updates (if you have a setting to install them automatically). So, it would be wise to switch PC on at least once a month only for maintenance in a day after important updates are released, doing nothing until the updates complete.
In particular, whether a task will be run immediately after the schedule is missed, depends on a setting in the Task Scheduler. You can open TS and look at this setting in detailed view of any task, Settings:
[ _ ] Run task as soon as possible after a scheduled start is missed
Whether this option is ticked, depends on developer's decision. You may change it, but better to leave as is.