@Tanvi Desai - Usually most of the companies keep a payout period of minimum 3 months i.e. paying out 3 months after the candidate joins. However, if you might want to be generous and more impactful, keeping the payout period shorter is something I recommend. We follow paying one month after the candidate joins, at Rocketium. To keep it easy for our finance team we don't split it into instalments but I liked the idea that Rohan has shared above, that can help in retention.
In terms of the amount - IMO, be as generous as you can. While deciding our referral incentives we looked at how much we would have paid a vendor/agency if we hired from them.
For e.g. if you were hiring someone at 20L CTC and hire via vendor that charges minimum 8.33% also, you'll end up paying 1.66L. So even if you give a referral amount of 1L to your own employee, that still saves some cost for you and also makes them happy + encouraged to give more referrals 🙂