I had a week on the Gold Coast (from the Sunshine Coast) for my husband's work about 2-3 weeks after I finished treatment. I was pretty good at that point but still felt a bit fragile. I didn't do much - just lounged about in a nice hotel while he conferenced. There were lots of dinners and functions - most of which I managed to attend but a few that I skipped due to tiredness etc. I would have been disappointed if I had paid a lot of money for that trip (as a holiday) but it was better than being home by myself as my husband had to be there anyway. I like to do a lot of activity on my holidays and I get bored sitting around.
3 months after finishing my treatment I traveled to Thailand - by then I was fine. It was a family holiday and we all scuba dived, hiked, kayaked, rode bicycles and generally had a fabulous time. I had a few aches and pains (I still do - thanks to chemo and Arimidex!) but I was as fit as I had been prior to my diagnosis.
I think most people recover from radiation within a week or two of treatment finishing but everyone is different. Some people breeze through with barely any issues and others really struggle. What type of holiday are you hoping for? If it's not too physical you should be right within a few weeks. If you're traveling with someone who can carry your luggage and you have a nice relaxing time planned then it should be wonderful.