If you're going to be in Granada soon, the ticket system is changing. The ticket retrieval machines at Corral Del Carbon had a sign that they don't work, and a helpful worker there confirmed it.
Our palace ticket time was for 4pm. We took the handy C3 bus over to the Alhambra at 11am, retrieved our tickets, walked down to the Justice Gate to walk right in and explore Charles V, etc. We ate a lunch at the American Cafe, toured the palace and walked up through Generalife, finishing around 6pm.