That is always a very interesting question. When is the right time to apply for training ? The genuine answer to that question is that there is no definite time period for application which will grant you an internship in a good architecture firm for certain. In today's age, when there are so many architectural colleges and so many students competing for those few internship opportunities, it becomes highly important to apply early enough so that your portfolio will be reviewed properly. Most of the firms, especially the really good ones, usually finalise their selection of candidates for internships quite early itself and don't leave it till the last moment.
The earlier you send your architecture portfolio the better. This is especially so because, this will give the firms a lot more time to go through your portfolio in detail and if it is good then block you or particular seat before considering another applicant. And in my experience, the portfolios send In earlier, which are relatively decent will always be be selected initially itself and the decision will not be reversed.
Some of the firms, like Rahul Mehrotra Associates, clearly ask for you to apply at least 4 months prior to your date of joining. That means that you have to start preparing for internships minimum 6 months prior to your joining date. You will have to do your research and identify the architecture firms you want to send your portfolio to. Once you have your list of firms ready, you should immediately send your architecture portfolio and resume. You should ideally have a good number of firms identified to send your portfolio. You cannot decide that this is the firm that I want to work, so I'll send it here only. This is because, getting selected for internship in your choice of office is very difficult. So always be safe and apply to as many good practices as possible.