We get a lot of questions about whether certain signs are legal or not. Here is a summary of the some state and county laws, but see the statutes (ORS 377.735 and ORS 377.720) for full information.
Rules vary by city. City regulations generally trump the state rules within the city limits; consult each city's planning department for their rules.
The county follows the state rules.
Information summarized here, but check the jurisdiction for up-to-date information (see links provided below).
Note that most ordinances refer to “the election” for the dates. In Oregon, the “start” of the election is considered the day that the County Clerk mails ballots an the “end” of the election being the day ballots are counted. So “90 days” or “six weeks prior” mean 90 days or six week prior to the day ballots are mailed to voters.
It is not clear that laws limiting temporary political signs by time around an election specifically would be constitutional under the US Constitution's First Amendment protection of Free Speech: it would be discrimination based on content: politics. But the laws probably would pass muster if they were applied generally: only N days before and Y days after an event of any sort. Courts do generally allow "content-neutral and reasonable 'time, place and manner' restrictions on speech" -- size, shape, location, ... may all qualify.
Oregon
No signs allowed in the state right-of-way. Temporary political signs are allowed on private property within view of state highways with the following restrictions:
* Signs must be 12 square feet or less (3x4 is usual) when visible from the state highway unless the state granted an exception (very rare) and cannot be in the state right-of-way.
* No flashing or intermittent lights, animated or moving parts are allowed.
* Signs must not imitate an official highway sign or device.
* Signs are not allowed in scenic corridors.
Call ODOT and they will remove signs in the right-of-way; they will send a letter to the property owner if violates provisions such as size or duration (60 calendar days per year max).
For questions, or to request a size variance, contact Amy Joyce, ODOT Outdoor Advertising Signs Program Coordinator at 503/986-3456 or amy.b.joyce@odot.state.or.us. To report a violation in the state right-of-way, contact Dave Cabbish, the local ODOT, contact at 503/474-1462 for removal.
Yamhill County
Yamhill County follows the state rules; report violations to the County Planning Department which may issue a zoning citation if it is on private property.
“Temporary political signs advertising candidates or issues may be erected on private property, with consent of the property owners, during an election campaign. Such signs shall conform to the requirements of the State, and shall be removed within one week after the election.” Contact the Department of Planning and Development at 525 NE Fourth Street or call (503) 434-7516 or email planning@yamhill.co.or.us [as of 9/14/2008] http://www.co.yamhill.or.us/plan/planning/ordinance/zoning_1006.asp
McMinnville
McMinnville's sign ordinance was extensively updated in 2009. The sign ordinance is now "content neutral" so that political signs are not specifically referred to; they are regulated as all other "temporary" signs. Normally one is restricted to two temporary signs at a time, but from six weeks prior to an election to 14 days after an election, residents can have an unlimited number of signs. Not to exceed six square feet in area. Contact the Planning Department at 231 NE Fifth Street or call (503) 434-7311 [as of 3/24/2009] http://www.ci.mcminnville.or.us/images/stories/departments/Planning/misc/signord_amended_by_%20genregs.pdf
Newberg
1 sign (any sort, political or business, ...; therefore "content neutral") allowed from 90 days before the election, not exceed six square feet, removed within 14 days. Contact: Planning and Building Deparment, City of Newberg at (503) 537-1240 [as of 7/16/2008] http://www.newbergoregon.gov/sites/default/files/Election_Sign_Information.pdf
City of Yamhill
http://www.cityofyamhill.com/amc/10064.htm
City of Lafayette.
http://www.ci.lafayette.or.us/Sign%20Code.pdf
"Temporary signs or banners that do not exceed 32 square feet in area. No lot may display temporary signs for more than 90 days in any 365 day period. Only one temporary sign per lot may be displayed at a time."