Right now, for the time being it is in the account hivetips.pool however, this is already being coded in for the next update to the app whereby if the tip is not claimed withing 30 calendar days, it is returned to sender.

30 days has been a norm used on Hive in the past and it is ample time for people to read the comments on their Tweets or read their Notifications on Twitter.

Great question, glad you asked that question, love getting the info out as to what is in the works.