Question

Just wondering if a Timer (Threading) will finish execute his task before the waiting period for the task re-execution ?

Answer 1

See Remarks in System.Threading.Timer:

The callback method executed by the timer should be reentrant, because
it is called on ThreadPool threads. The callback can be executed
simultaneously on two thread pool threads if the timer interval is
less than the time required to execute the callback, or if all thread
pool threads are in use and the callback is queued multiple times.

So for System.Threading.Timer the answer is No.