How long do you have to wait to get a crown after a root canal has been done?

Poor you! Dental problems always come at the wrong time, especially over Christmas! The reason for not crowning the tooth straight away is to make sure the root filling has worked ok. Sometimes a tooth is dressed more than once before the final root filling and even after the final RCT the dentist will want to ensure that all is well before placing a crown. I expect you have a temporary crown or filling in the tooth and that's the reason for advising a soft diet. Unfortunately, things in life do tend to be unfair - all I can suggest is mashing your food with a fork and sticking to pasta, fish, etc and cutting up anything else REALLY small. I know it's Christmas but you could always make up for it afterwards by having a really good meal in the New Year after your crown is fitted (but steer clear of toffees or you will pull the crown off!!)