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.
Some people have been giving the detail about when the snapshoot will be taken. So many holders prepared their XRP on exchange site to did a flash dump to the XRP market. It seems like the impact caused by the snapshot already appeared. The market will be turning into the bearish trend. Every XRP holders must be careful with this.
This could be a big flash dump for XRP because they whales will be liquidated their money.