Help with a custom weapon...

[Zeiss]

I want to make a custom enchantment for a player in my game for his weapon. He's wielding a spiked chain and i want to give it a uses/day ability to increase it's reach by 5 ft. What is the best way to go about doing this?

I was thinking either 3/day for 2-3 rounds each use. Or 5-8 rounds a day used whenever he wants them.

Party is ECL 5 and running in the Red Hand of Doom adventure.