In test match format new ball can be taken after 80 overs. My question is that if bowling side captain don't want to take new ball after 80 overs then how much can he use old ball
He can continue to use the old ball until either he chooses to take the new ball, or until it's declared unfit and is changed by the umpires. However, it's supposed to be changed for one of similar age, so that will complicate things. See Law 5.
In practice a captain will use an old ball if it's working for the spinners, but eventually it will go out of shape, and it will be worth it to take the new ball, if only to give the spinners a rest.
The captain can use the ball for as long as he wishes, so long as the ball is fit for play. If the ball is unfit for play,
the umpires shall replace it with a ball which has had wear comparable with that which the previous ball had received before the need for its replacement
if the ball needs to be replaced after 110 overs for any of the reasons above, it shall be replaced by a new ball.
This provision was brought in after a couple of incidents where captains were continuing to bowl with very old balls, and it was becoming increasingly hard to find a ball with "comparable wear".