I did my instrument training over about 3 weeks, I guess (too lazy to break out the logbook). It can be done in alot lees time than that, with a few conditions already being met.
How much cross country, PIC time do you have? How much simulated instrument time? Although you could do it, I'd hate to try to cram 50 hours of flying into two or three weeks.
The total immersion concept works really well, but there are some other things to consider. For example, if you take 6 months to get your instrument rating, you're going to have most of the information burned deep in your memory for the rest of your life. If you get your rating in 3 weeks, the same won't be so. It's going to take alot of effort on your part to keep the same level of currency (initially). After you've gained some experience in the system, however, the end result is the same.
I'd say go for it, if you're going to be able to follow up with alot of practical, real-world experience. I'm a fan of condensed training - my biggest problem is finding an instructor that's willing to do it!!