There are several trains each day. The best of them is the Ajmer Shatabdi, a 'luxury' express. Luxury because (1) it's fairly clean, (2) it doesn't make a lot of stops, (3) you're served a light breakfast, and (4) it has only two classes of service, '1A' and 'CC.' The train leaves the main Delhi station at 0610 and arrives in Jaipur at 1045. (Correction: it's supposed to arrive at 1045. When I took it, a little over a year ago, it got in about 1130.)
Fares are Rs905 (about US21) for 1A, 465 ($11) for CC.