In case you hadn't noticed, selling a home is taking longer today than it did a year ago.

A Better Homes and Gardens Real Estate Service housing-cost survey found that the national average number of days a home stayed on the market was 105 for the first six months of 1990 - eight days longer than the same period reported in 1989.Homes in the Northwest (not including California) were sold an average of 32 days faster than the previous year, according to the survey, 67 days on market compared to 99 days in 1989. The survey reported homes in Bellevue, Wash., a Seattle suburb, selling in an average of 18 days.

The Northeast and California showed a real estate slowdown, with the average numbers of days on market doubling for California, from 44 in 1989 to 87 in 1990. In the Northeast, homes averaged 143 days on the market, up from 127 days in 1989.

The annual survey was based on reports from participating members of the Better Homes and Gardens national network of 700 firms and reflected the average sales prices recorded in each market for a sample home described as an upscale, three-to-four bedroom, single-family detached home in a desirable neighborhood.