For what length of time can i leave a glass ionomer cement in my completed root canal treatment?

Confused: Where is the gi. The actual canals should not be filled with GI but gutta percha and cement. If the top of the canals is sealed with GI or even as a base up that can be ok. If you mean there is no final restoration and just a GI build up, you need to get it done before it may fracture and the whole tooth may need to be extracted. Unfortunately there is no timetable, do it asap to make sure.

Crown : It is best to get a crown on the tooth asap. As far as how long you can go before the crown is done depends on where in the mouth the tooth is and how much of the tooth was compromised before the root canal was done.

DONT WAIT: Dont test the longevity of a restoration on a root canal tooth especially if it a molar. These teeth have a high chance of breaking and you already paid a handsome fee to save the tooth. Get your crown done asap. 6 months is the longest i would wait but i would try and have it completed within 30 days of root canal treatment.