Always look up or call pharmacy about any IV push med you aren't available. Also, a pt's response can determine if you slow down/speed up. For example, I've had pt's that become hypotensive to IV push narcotics, so I've been known to push over 2-3min if it helps prevent the pt from becoming hypotensive.
Things like Protonix or Solu-Medrol can be pushed faster. BP meds and Lasix I tend to do slower (I seem to remember a rule from nursing school
being for each 10mg of Lasix take 1 min to push).
Also, the type of line you are pushing into can matter. A 24G peripheral can not have things pushed into as fast without burning or irritation as compared to a central line.
Oh and I always dilute IV narcotics and sedatives in NS. Zofran technically isn't supposed to be diluted, and some BP meds can work slower if diluted.