By month six you have enough credibility to push back, the question is whether you're using it. A team that never says no to anything from above isn't protecting its time, it's just quietly absorbing everything until something breaks.
New managers often equate saying yes with being a team player, especially early on while they're still proving themselves. By month six that instinct needs to shift. You have context now that you didn't have on day one, and part of the job is using it to push back on asks that don't actually hold up.
Distinguish "hard" from "not actually necessary"
Some pushback is about difficulty. The more useful pushback is about whether something needs to happen at all.
Say no with a reason and an alternative
"Not now, but here's what I can do instead" lands very differently than a flat refusal.
Notice if you're the only one who never pushes back
If every other manager at your level pushes back sometimes and you don't, that's worth examining, not wearing as a badge of flexibility.