This Arnie Baker ebook has great interval suggestions, routines, etc. http://www.arniebakercycling.com/books/b_hit.htm

According to Arnie, 4 minutes is a really effective interval time. It really helps to have a heart rate monitor and know how to use it (i.e., what your target zones are by something other than 220 - your age). You would do speed or harder gear or hill climb or something for 4 minutes, sufficiently hard to get you into zone 3 or 4 (probably just zone 3 for a real beginner). Without a HRM, this is an effort level at which you cannot speak, period.