I think it'd depend on the situation. Did your agent give any indication of how long it'd take them? How long has it taken them in the past? Is an editor waiting for the manuscript? All of that would influence the time considered 'reasonable'.
Also, from the sounds of it I'm sure your polite nudge would genuinely be polite, but just in case it's helpful, I tend to frame these nudges as, "Do you have an idea of your time frame for reading? That way I know what to expect and can adapt my schedule accordingly." Like, if I know my agent plans to finish it in two days--time for a TV binge! If it's a week or two, well, I can embark on beta reading and making notes on future ideas. One or two months? Time to write a new draft, or edit an older one.