See if there are realistic alternatives. Lamictal is not a realistic alternative for a person who has a history of severe mania who is also likely not going to be compliant if they don't get the labs done. An antipsychotic could be.
If they don't get the labs done, and we got time, I tell them they got to get the labs done. What I mean by we got time is if it's the three-month period, they didn't get it done, I'm not going to lose sleep over it if they get it done within the next month.
After that it depends (for me). If otherwise healthy, I allow it to stretch to 6 months while documenting that they are not following my recommendations and have already been warned of the risks. If past 6 months and there are no alternatives, I terminate the patient/doctor relationship. I cannot treat a patient on Lithium or Depakote where they will not get the labs done for an extended period.
If the patient has health issues, e.g. already has some renal problems and is on lithium, I will likely not allow them any grace period. For you greenhorns there are sometimes situations where one still has to be on lithium despite renal problems.
Warn patients ahead of time of these things. I've noticed when this happened, the majority of them won't freak out at you because they understand why you did what you did.