Some of the abilities in GURPS are priced in uses per session, but not all abilities are priced that way. This can lead to some oddities when creating a character who have abilities that are mixed type. Most abilities are either “always on” (e.g., they are active at all times for the character) or “switchable” (e.g., they must be switched on and a cost of some kind paid). For example, Combat Reflexes is always on, where as Warp must be activated. This is further complicated when some abilities fall into uses per session (e.g., Serendipity) or uses per hour of game or real time (e.g., Luck). So how to reconcile this and make abilities used in a more general way? Well, the Game Time modifier does a lot of the heavy lifting for that really. You can turn uses per hour in game to real hours and vice versa. But what if you want an ability that can be used an unlimited number of times? Or that is powered by unspent character points or Impulse Points? Here’s one method:
To turn an ability from uses per session into you need to figure out how many times you can use it per real world session. Then, add Game Time to it. Next, add a number of levels so that it works at least once per game world day and then multiply the final value by 1.67 (this is akin to buying off Limited Use, 1/day on the ability).
Example: Serendipity 1  works once per real-world session so we add Game Time to this so that it works once per game-world week. Next, we buy enough levels of the trait so it works once per game-world day (in this case, seven levels of Serendipity). That’s a totally of 105 points. Next, we multiple this value by 1.67 for a total of 175.35 points, the GM rounds this down to 175. “Serendipity ∞” costs 175 points and can be invoked at will.
Abilities which already have leveling schemes (e.g., Luck) should use those instead as well as the Game Time modifier to get them down to less than a second for “unlimited use.”
Picking Over the Bones
Unlimited uses might not be appropriate for all campaigns and its suggested the GM put some sort of limiter if they feel they need to. Having an unlimited ability require character points per activation is a good way to limit it while letting characters use it whenever they feel like it. Using our above example, Serendipity ∞ (Costs Character Points, x1/5 pts) would cost 35 points and could be activated whenever the character wanted by spending a unspent character points, Impulse Point, Destiny Point, etc.