Need to "gift" an app purchased in google play by issuing a refund later. How many days can I wait to refund? I know Google play still doesn't support gifting of apps but I need to provide one of my clients a free copy.

They already purchased it. I am going to refund them later, after they are done using the app. How many days from purchase are valid for issuing a refund? I can't find this answer online. I see some other purchases made >30 days ago that are refundable, but I don't know if I can wait 60 days? 90 days? 120?

Can someone login to their Google Wallet and see how far back the refund button is available?
The refund button on the Play store is only available for 2 hours (although Google's support pages say 15 minutes).

From your question it sounds like you may be the developer of the app, in which case you can manually refund the purchase regardless of when the app was purchased via your Google Wallet merchant account. I don't think this actually reverses the purchase (they can still install and use the app) but the cost of the purchase is refunded to the user.