The Magic Item Creation rules are guidelines.
The most important thing to remember when making magic items in Pathfinder is that the rules for making items are guidelines, not hard-and-fast rules. Making new magic items is a very subjective process, and no set of rules is going to give you balanced items each time. The only way to reliably price new magic items is to make new magic items and price them, and see how effective they are in play. Experience is much more important than rules, here.
The base price for command word or use-activated items is for at-will use.
Command-word and use-activated items are, by default, usable as many times per day as you want. There is no limit to the number of times you can use that kind of item in a day.
As an example, the Hand of the Mage. It lets you use mage hand as a standard action, as much as you'd like. It costs 900 gp, which is spell level (.5) times caster level (1) times the command-word constant (1800) for an even 900 gp.
Very few command-word items exist that don't have a per-day limit to their use, because at-will usage of many effects is significantly more powerful than just being able to use it once per day.
Always check for similar effects when pricing at item.
This is the second most important rule of magic item creation. A large number of effects are already represented in the existing magic item rules. If the item that you want to make duplicates the effect of an existing item, then it should cost the same as the existing item.
For your example of an item that lets you cast mage armor you should compare it to existing items that do the same effect, like the Bracers of Armor. The +4 Bracers give you a +4 armor bonus to AC all day long, with no activation needed, and cost 16,000 gp. Your item gives you a +4 armor bonus to AC for 3 hours a day, and needs an activation before it can be used. Clearly, your item should cost less, but exactly how much less is something that's more left to how the DM feels about the item's cost than any hard rule. Personally, I'd go with around 8,000 gp. If you're having about 4 encounters a day, and you have the ability to more-or-less predict half of them, then you're getting about half the benefit that the Bracers of Armor +4 would give you, so I'd charge about half for the item. But that's just how I would run it, and it's more important that you price it in a way that makes sense to you.