I don't know what tier you are in or champs you are using, or how long you have been playing, however the idea of the defensive team is not to lose, you hold the cards as they play from AI - basically you're picking arena teams with revivers and your team isn't strong enough (damage wise) to take them out before they revive

So, the longer you play the easier it will be to recognise a team composition and decide if you can win with your team, it will, or should cease to be an issue for you

Even so, wouldn't it be clever to stop the game after a given amount of time if the attacker clearly "can't win"? Just don't get why there's no time cap...

Hate  it  in  tag  where  im  not  sure  if  ending  the  battle  will  skip  the  round  or  forfeit the  whole  thing.  

Wouldn't it be possible to set a time limit of 5 or 10 minutes 

Just don't get why there's no time cap... 

There's a turn limit of 1500. You reach it you lose.

arena battles prologed unneccesarily 

It is VERY necessary to prolong these battles - for the defender. If you attack my go second team I want you to get bored and concede or even wipe your team out by attrition. 

I think it should be left as it is, specifically because of the reasons n00b listed. We want to dissuade attacks as much as possible due to gaining nothing from winning our defenses other than staying put. 

I think it's also a valuable (albeit annoying at times) lesson to learn on what kind of champs/comps you should be avoiding if you're lacking certain answers for them. Then we can go about figuring out how to beat those lineups, or if we should just be avoiding them altogether until we find champs that counter them.

As attacker, you can stop the battle at any time manually. Why dooes it need to stop automatically after 10 minutes? If you see, that you can't win, just stop the fight. If both teams are at 99% of hp at this moment, so they can't beat you either, bring a champ like Vlad the Nightborn and try again.

FYI, you can abort the battle and take the loss whenever you want.  Just hit the pause button in the corner and hit end battle.

Set to 10 and win that team which has more hero or more team power at end of time.

Nope, find a team you can beat, if you can't the defence did it's job

When skipping in Tag you get the choice to skip the battle or skip the whole series