One of the fundamentals of singing is to understand that consistency is a super important key to singing better.
If we only go to the gym once in awhile, then we certainly are not be able to get fit. Same thing with the voice. If one does vocal exercises willy nilly, a little here and little there but never consistently, progress will not be achieved.
Now, an important caveat. Exercising consistently is needed for progress. But, if one were to consistently exercise incorrectly then this would be very bad indeed. For practice makes permanent not necessarily perfect.
So, we need to exercise both consistently and correctly!