In this relevant forum thread, Fred Hicks from Evil Hat comments:
Free invokes arise from situations. They last as long as the anchoring situation does.
From the rules on situation aspects (emphasis mine):
A situation aspect is temporary, intended to last only for a single scene or until it no longer makes sense (but no longer than a session, at most). Situation aspects can be attached to the environment the scene takes place in—which affects everybody in the scene—but you can also attach them to specific characters by targeting them when you create an advantage.
Who can use a situation aspect depends a lot on narrative context—sometimes it’ll be very clear, and sometimes you’ll need to justify how you’re using the aspect to make sense based on what’s happening in the scene. GMs, you’re the final arbiter on what claims on an aspect are valid.
The upper limit for situation aspects is a session, rather than a scene. The second paragraph talks about who can use a situation aspect, but could also apply to when one can be used, as well. The fictional context will determine how long they remain available. So if it seems like the party has too many prepared advantages available to them, shift the fiction somehow.
In your heist example, there are many reasons in the fiction why the characters cannot just prepare indefinitely. Preparing takes time, and there are only so many hours in a night. The loot might be time-sensitive; maybe it is going to be transferred to a different location tomorrow, so the heist must take place tonight! Maybe the party's employer will get impatient. Maybe the security pre-hack will be discovered before the party arrives. If the party asks around about the building in question, maybe the building's owners will get tipped off. Maybe the building is getting a security upgrade soon and the research won't be up-to-date for very long. If the player characters recruit help from someone, or find a guide or traitor to help them break in, maybe the NPC is only willing to help them tonight. Maybe the party's extensive preparations give the enemy time to prepare as well.
Discuss your concerns with your players. Getting on the same page with them about this may lead to them self-policing themselves.
Finally, this answer to a different question, particularly the part about the "blind sniper," touches on the issue you are facing. To summarize, Fate rewards building convincing narratives. A small group of player characters, outnumbered and relatively unequipped, breaking into a well-defended building with cunning and planning and teamwork is a compelling narrative. It might not be as big of an issue as you think.