A DUI conviction is not eligible to be vacated under RCW 9.96.060. Therefore, a DUI will remain on your record indefinitely. However, for purposes of prior DUI convictions it can only be used for seven years. In other words, if you are charged with new DUI and your first DUI falls within seven years of the new charge, the first DUI will be counted as a prior offense and you will be sentenced under the 2nd DUI offender guidelines. This also applies to a 3rd offense. It is a class C felony if you have four prior DUIs within a ten year period.
In an effort to expand on the previous answer, it seems that the question was how long does a record of ARREST stay on ones record. An arrest alone does not constitute a criminal record. A criminal record would occur only after a finding of guilty after trial or a plea to a criminal offense.
Unfortunately, a DUI conviction will remain on your record forever; however, from your question it appears that your DUI has not been adjudicated fully. If your DUI is reduced to a lesser criminal offense then for sentencing purposes it will affect you for seven years. If your DUI is dismissed, you can move to have that non-conviction data removed from the court's record. Caveat: the WSP, prosecutor's and judges will always refer to your record, even if it didn't result in a conviction at all.