Don’t leave it too late. Conference interpreting is a very specialised skill. There aren’t that many good and professional conference interpreters on the market.
Furthermore, most events tend to be organised at the same time. So if there are several other conferences in the week you want to organise yours, the chances are, all good interpreters will already have been recruited.
Once they have found good interpreters, most regular conference organisers tend to book them long in advance.
You will be competing with them for the same limited number of professional interpreters.
Don’t think that by leaving it till the last minute you’ll be alright or you’ll save money.
You may end up paying more– because you will need to import interpreters from elsewhere – or having to compromise on quality.