If they haven't accepted the contract, you can always cancel it.
If they accept the contract and keep the horse, it's kind of a grey when they've had enough time. Some people can be on every day for hours at a time and a week would be reasonable, where others maybe can only get online on the weekends so require longer. It also depends on what was discussed with them.
Make sure the player has been online. I've had one instance lately where something similar happened and come to find out, the player's computer crashed and they were unable to get online for a couple weeks. They certainly didn't intend to ignore anyone.
You are always welcome to report problems at
http://www.orbisworlds.com, if you feel it's taken an unreasonable amount of time. We can attempt to contact the player as well and ask for their side or what is going on.