If the trees are still dormant, you will need a certain number of evenings above a particular temperature to begin the budding process. That cumulative effect could depend on the grower's climate too. If it were warming up there, the budding might happen sooner.
In any case, I'd expect to see some bud movement within a month to six weeks. Maples can be very tough to grow in a desert climate. You might want to check some of the Vegas area club websites and look into shade cloth and/or mist, humidity trays, etc. Anything that will keep the humidity up, temps down and leaves on!
If you aren't already a club member, consider joining one to learn about what people are growing in your climate and HOW.
Here's a desert bonsai FAQ:
http://www.phoenixbonsai.com/PFAQ.html
Regards,
Matt