2 - 4 horas
I didn't want to pay Apple's high upgrade price for an SSD so I ordered with the base 500GB drive with the intent of keeping it as the secondary data drive and installing my own SSD for the system drive.
I followed the guide and it went exactly as described with no issues. The procedure took about 45 minutes.
Instead of re-installing Lion or using Lion Recovery, I used Carbon Copy Cloner to copy the original disk to the new SSD and then set the SSD as my boot disk. This only took about 20 minutes and kept my system in exactly the same state. If adding your own SSD, also look up how to enable TRIM in Lion as it is only enabled by default if you order one from Apple.