There is no specific time range given for the time between preparation and immersion. Even so, the halachic preference is for them to be as close together as possible whenever such timing does not compromise the thoroughness of the preparation.

It is customary for a woman who prepares at home for immersion – at any time that day – to take a quick shower, comb her hair again, and visually inspect herself at the mikveh prior to immersing. This does not take long and would be the proper procedure in the scenario you describe.

The mikveh attendant is present to help women keep these halachot, but does not have halachic authority over a mikveh or its users. If there is ever a disagreement of a halachic nature, it is best to ask politely to call one's own rabbi or the mikveh's supervising rabbi.