Ideally 11-12 hours
When a med is given twice daily or 12 hours apart is best in term of pharmacokinetic, but real life chaos makes it virtually impossible to do so regularly for long period of time.
So, if you have to take it 6 hours or 14 or 18 hours later, take it when you can. Over time, it does not matter much because of a build-up of medication in your system. Ideal vs real life, do when u can. Good luck.