there is likely to be some fluctuation, but only minimal. The whole point of stand development is that it is self limiting and compensating in nature. So, as the highly dilute developer exhausts in the highlights, it continues to work in the shadows bringing the negatives into an acceptable contrast for printing. When you are talking about hours/ minutes instead of minutes/ seconds, it is much less critical.
As there are also so many other variables in developing, it is always best to work out your own times ... having to use grade 5 too often? Develop for longer (and vice versa). Having to force the s-curve too often in photoshop? same again.