All Mega Stones are non-disposable items, which means yes, you will be able to Mega Evolve it in every battle (given that no other Pokemon Mega Evolved first, as you may only Mega Evolve one Pokemon per battle).
The Venusaurite or any other Mega Stone will not disappear after one use like items such as Air Balloon or Weakness Policy, so you have nothing to worry about.
Of course, you will have to press the button on Venusaur's move screen in battle to trigger Mega Evolution. All Mega Pokemon go back to their original state once the battle ends, so you will have to manually press the button to make it Mega Evolve every battle.