7 days of inactivity on mine. iPhone 5c. If there is someone whom you know would never decline a game from you directly and it says he/she did, it is more than possible they have a lot of open games already that they might be trying to finish to clear them out and earn the rewards ie. scratchers. So if they have a lot of these half finished games they are trying to finish and have not had a chance within 7 days to get to the game you sent him/her and that 'no turns have been played' by him/her the software will automatically send it back to you saying the game was "declined". More likely this person just didn't have an opportunity and/or the time to play the first round within the 7 day period. Could also be he/she just never opened the app in those 7 days due to being busy with work etc.
Hope this helps.