The best time to leave a property is upon expiration of the lease term, but there is always room for negotiation.
Several months ago, a client (the tenant) had a lease that was close to expiration. The tenant was about to sign a lease for a different space, but something happened: Another property came on the market, they looked at it and decided it was much better suited for their business. However, starting all over in the negotiation process would have put the tenant over their current lease expiration date. In order to acquire the new space, the tenant would have to extend their current lease, or pay a penalty.