Can a Tooth That's Been Prepared for a Crown Keep Getting a Crown on It?

Fortunately, crowns are among the longest lasting restorations we have, and you may find some last up to 20-30 years.  Some, much less.

There are many factors to longevity of crowns, not limited to if a person clenches, grinds their teeth while they sleep, hygiene habits, periodontal condition, material of the crown, and so on.

Each time the tooth is worked on, some tooth structure is removed.  It is a good idea for the dentist to be as conservative as possible to allow future work if needed and to avoid the pulp/nerve.

At age 19, you are wise to be aware that these are not the last crowns these teeth will have, perhaps 2 or 3 more times.  It is good that they are only 3/4 crowns now, which will allow future work to be done.

I think that the answer to your question depends on several factors.  Do you see a dentist regularly for cleanings and exams?   Is your hygiene excellent including flossing daily?  Do you have an ideal bite? Are you a grinder?  All of these things play a factor in the lifespan of a tooth after having a crown.  Typically, a crown can be replaced when the existing one breaks or gets recurrent decay.  Only if the problem has been present for a while before seeking treatment are you in jeopardy of losing the tooth and then needing an implant.  Hope this helps.  

Crowns can be remade on a tooth indefinitely as long as there is tooth structure to attatch them too.  They can last many years as long as you take care of your teeth and get your checkups.  One should always keep restorations to the minimum size so that when they need to be replaced that there is enough tooth to continue to use them.  Sounds like the 3/4 crowns were a way to be more conservative and save more of the teeth and that is good.  Always keep your restorations small.  There are so many ways to keep things small nowdays.  Talk to your dentist or find one that believes in minimally invasive dentistry.