In Secrets of Practical Chess, GM John Nunn advises players not to spend more than twenty minutes on any move. While this is sensible advice, positions will inevitably arise in your own games that demand at least twenty minutes thought. Cutting your calculations short in order not to fall significantly behind on the clock is what makes this 'practical chess'.
However, by observing the discipline Nunn and others promote, you risk missing strong possibilities. While Nunn's advice is good in the context of practical chess, no aspiring player should be afraid to lose themselves in calculations. This, after all, is the heart of the game.
Nobody can calculate like a computer, and complicated variations have to be calculated over and over again and the resulting positions evaluated and re-evaluated. Before you know it, in a strategically or tactically rich position you can and will have used more than twenty minutes on a single move.
Getting lost in thought is part of the fun of chess.
But should you feel time trouble, or simply time management, is needlessly costing you points, then you could do worse than follow the Soviet methodology, as practiced by Kasparov and Nikitin (his trainer), of performing a 'chronometry'. Already you may have observed players marking their scoresheets with notes as to time spent on each move. (NB: This practice is now illegal in tournament play, in which all excess writing on scoresheets is considered a potential aide memoire). This is to later analyse where their time is being spent with the hope of identifying a pattern of excess time usage at a particular point in the game. A pattern that may then be altered to maximise results.
Generally speaking, if you look for such a pattern in your time management then you will find one. You will not need to plot graphs as the former world champion and his trainer did in order to recognise it. If you have teammates or a coach, then they will probably already be aware of it. You may only have to ask them.
With regards to the conclusion of any game, whether increments are in operation or not, always maintain a positive and calm disposition by reminding yourself that five minutes is enough time in which to play a decent game of blitz chess. Do not panic. Throw yourself into the task at hand, abandon the quest for perfection, and play the remainder of the game as if it were a blitz game.
Do not panic.