Depends on alot of variables. I have kept my pair from COA in a 150 for a year. In that time they grew from 6" male to 13" female from 5" to 9". I changed their water religiously 100% every week. Fed them 1 time daily and periodically skipped feedings throughout the week. They still grew fast.
A 180 would be the minimum I would keep a pair for life. A 30" from front to back would be ideal. A 180 will get you two years at least maybe longer I would say once he gets over 20" it's time to get a new tank.
These fish can't be truly appreciated in small tanks they should be experienced in massive tanks to get a idea on how they move in open water. Not everyone can provide accommodations like that. I can't currently but it does take time to grow them out.
Drew