As a general rule of thumb, you should start planning for your bachelorette party a couple months in advance. This will give you enough time to contact all of the potential guests, have a few potential date options, and figure out who is available and able to attend the party. It will also give you time to book any venues, vendors, or flights/hotels that you may need. However, if you have a specific date or weekend in mind for the party, you may need to start planning even earlier.
For example, if you want to have the party during a popular summer weekend, you may need to start planning six months in advance. This is especially true if you are planning to travel for the party with a larger group of people. Popular summer destinations can book up quickly, so you will want to make sure that everyone has their accommodations and flights booked well in advance.
On the other hand, if you are flexible with the date of the party, you may be able to get away with starting the planning process a little bit later. If you have a few potential dates in mind, start by contacting the potential guests and see when they are all available. Once you have a date that works for everyone, you can start getting things rolling and get everything booked.