how long can a motor last while knocking?

 

Not to sound flip in my responce, but it makes a difference what engine and whats' knocking. I'be seen some (Chrysler slant six) which would go years with a rod knock that would loosen teeth and yet others (GM 60d V6's) would blow out the bottom end with no warning what-so-ever.
A quick rule of thumb;
Knocks caused by valve lash adjustments and carbon deposit build-ups can be addressed without too much trouble.
Knocks caused by anything else are in effect like hand grenades with the pin pulled, you just don't know when it's going to go off. They require attention because the underlying cause always fails at the most inconvient times.

 

which engine is it?? 4 or v6..
like p-adams said it depends where noise is coming from upper end or lower end. if a i4 engine .is noise there when first start and then it goes away and coming from bottom end most likely piston pins it can go as is for long time ....if it is a v6 and noise is there when cold and gets quieter as warmed up could be hydraul lifter it also can go as is for a long time .....
you need to verify where noise is from

 

may stock f22 lasted a few months before blowin