The Shadow Tower has the effect:

Challenges: Kneel the Shadow Tower to add 2 to your total STR during any challenge in which you control at least 1 participating Night's Watch character.

How long does this effect last? I had always assumed it was just for the current challenge, but in a game the other night my opponent (Nick-ler) said he though it was for all challenges in the current round.

i think the word "any" is the key to this. it says any challenge in which you have 1 nw character with no clarification as to end of challenge or end of phase (or even end of round).

if i read the card right it not giving any nightwatch charter the +2 it give you the +2 to the challenge so kind of like the mealee titles that give you +3 sth in power challege only fi you have a charter in in that challenge

There is no "this" round limit, either. So why, if you kneel it in Round 1, does it not also give +2 in Round 2? And Round 3?

It is played as "current challenge," which is certainly the interpretation more consistent with these kinds of triggered effects. "During X challenge" is pretty much always treated as "current challenge." It could be read the way you are describing it, but if you do, then the lack of a "this round" limit becomes as much of a problem as the lack of a "this challenge" limit.

So the consensus is... treat as "This Challenge": and it won't continue to last - correct?

I certainly hope so.