I like to say, "my gut says X days. But I'll have to get back to you." I think there's a lot value in having the conversation with the PM or manager or whomever in the moment. And being non-committal is frustrating for the other parties.
I think what I intend to say with that can be, "I don't know how long it takes and it'll take more planning to figure it out, but I'm willing to participate in this conversation so that we can continue to effectively plan in the current context."
If the current context is "concrete" then you probably don't want to give a number. If the conversation is (and this was my assumption before) "here's a feature we want done but we're trying to figure out it's priority and feasibility" then giving a gut check adds value. You just need to be clear in your messaging.
And if someone uses a gut check number against you "but you said X days!" Then you need to call them on it and make sure you explained the discrepancy.