What makes sense for you depends widely on the nature of your business and the scarcity of your parking.

If visitors frequently can't get a spot, then you probably need more enforcement and/or more spaces. And if they are often short visits, 2 hours then tow makes some sense.

But sometimes a place will set a policy like this on general principle, and end up towing a visitor's car when its the only one in the lot.

As Jeff said, you can have people sign in and do away with the time limit. Or (if that is onerous for short visits) some sort of hybrid where you have the 2 hour spots, but a day long visitor can register inside and get access to a 'reserved' space elsewhere.

There's no best practice for something like this. Its all in the details. Some places shouldn't bother with a policy at all. Others can make do with ticketing or even warnings. But some risk losing business or having other problems if this is not enforced.