"No" can be interpreted as pure obstinence. "We can do X... I suspect we'll need at least N weeks after we have a proper scope/spec/X/Y/Z from you, though, so

"No" can be interpreted as pure obstinence. "We can do X... I suspect we'll need at least N weeks after we have a proper scope/spec/X/Y/Z from you, though, so if you want it done by $(MILESTONE) we need it by $(MILESTONE-N) at the absolute latest, or more preferrably by $(PREVIOUS_PLANNING_MEETING) so we have some wiggle room. No, that's not a spec. Yes, you need to participate in defining the scope of this thing - otherwise it'll take even longer when we're forced to rework completed things to work how you actually wanted them, and longer still when we work on things we thought you'd find important but don't care about. When can you meet so we can get this properly defined and fleshed out? We need a meeting anyways to discuss what other things you want that we're going to have to cut or defer to fit this into the schedule as well." If they work with you to cut/defer other things they've asked for and actually get you a scope/spec/X/Y/Z on-time, then good! You've successfully responded to a change in their priorities. If they suddenly lose interest when they might have to do some work... you've probably deflected some high work low value ask. Also good! If they put in the work but can't get you scope/spec/X/Y/Z in time... well, hey, that's not on you. You need enough time to get these things done. They might still want 3 women to make a baby in 1 month... but they might believe you when you say that's not possible if you at least look like you're trying to work with them on fitting X into the schedule.