Rule of thumb 3 to 6 months.

People in the fashion and, entertainment industry follow this rule religiously. I've worked in both.

You just always want to make sure you give yourself enough time four, planning and marketing.

Then during your marketing campaign keep track of what went well and what didn't it.

Then finally, after the season's over do a recap and plan for next year.

At the end of the day, just keep the marketing and planning process simple.