Without knowing the specifics of your start and end points, this can be a difficult question to answer. 880 (Nimitz Fwy) tends to have pockets of bad traffic during the rush hours. In particular, these are around Mission Blvd (where commuters from 680 are merging on), 237 (where commuters are exiting towards Mountain View) and 101 (where commuters are both merging on and exiting from).
If you have a flexible schedule, you will learn to time your commute (in both directions) to avoid the heaviest traffic. During some of the worst times, you can expect your commute time to double during the rush hour traffic.