@6mnth dental chkup dentist said xray showed decay around or under crown. expensive to replace crown. what is a safe time frame to save up and getdone?

Depends: It really depends on the extent of the decay. If the decay is close to the nerve, then delaying treatment may result in the need for root canal therapy, too, which would make your treatment even more expensive. Consider financing our care now.

Decay under crown: There is no way to know how much decay exists without removing the crown. However, the longer you wait, the more the tooth will decay, which could lead to loss of the tooth if the decay is not restorable.

Unknown: Impossible to tell what is going on under the crown. Much depends on the tooth underneath(all tooth, mostly filling,root canaled tooth,etc),and how old the crown is and is this the first crown for this tooth.Sometimes we get lucky and there is very little decay underneath,other times much of the tooth is gone by the time we get in there and a root canal is needed. Ask about temporary while saving.

As Soon As Possible: Even though a new crown might be costly, letting the cavity grow underneath a crown will inevitably cost you more. And, there's a good chance you'll also have pain if you wait. Worse, you might have pain and then also lose the tooth. That would be much more costly than replacing the crown now...sometimes 4 of 5 times as much work and money! So, get it fixed as soon as you can before it's worse.