Important if the value is the fact how long the opponent thinks before making a move? You look at this factor? and what information it gives you about your opponent?
agreed ^Yes, it is a useful tell, although sometimes there are extraneous reasons for a long period of time being used, and some players try to fake it.
I find there are actually quite a few spots where timing tells 'can' be quite useful. (I'd post a bunch of situations here (MTT) but then that'd be kinda -ev I'd think)how long they take is meaningless as far as tells go.
agreed ^I think how long it takes a person to act can give you valuable information, but it is not accurate for every situation. Sometimes people are multi-tabling, so you never know if they are taking a long time because they are just on another table making a decision. So I would say don't always rely on this as a tell.
It's opponent- and situation-dependent.
No. Becase lot of players are multitabling. So eg if they react slow, they just pay attention to an other table.
People tend to reverse time tell as a deception, relying on time tells is in my opinion a leak.