The best time to buy a straddle to catch the ramp before earnings is right after an earnings call in anticipation of the next earnings call. Be sure to buy enough time to capture the next earnings date. No reason to over think it.
This is way too early for few reasons:
1. IV increase rarely starts before 20-30 days before earnings.
2. The negative theta will start to hurt the trade right away.
3. If you go that far (~3 months), the effect of positive gamma will be pretty small.
4. For most stocks earnings date is not confirmed that early - usually it is few weeks in advance.