I have no idea when to take snapshot. But I recommend that you sell your xrp after taking a snapshot. Because I think it will dump. If you want to keep xrp in your hand later, you can collect it after it falls. But if you want to make a profit on both sparktoken and xrp, I suggest you sell it.
It will surely fall, the hype on Spark is over and XRP didn't even reach the price they were expecting due to the hype. I hope holding XRP for the airdrop will be worth it. Only 15% of what users are holding will be released after the snap and the remainder will be distributed over the next 25 to 34 months.