Kathryn Vasel

(CNN Money) -- Three weeks.

That's how long it takes to sell a home these days, according to the National Association of Realtors.

Five years ago, the median number of days on market was 11 weeks.

Low housing supply has pushed up home prices and created multiple offer situations and bidding wars throughout the country.

"The inventory shortage and the growing economy and job creation has increased the interest in homebuying," said Lawrence Yun, chief economist for NAR. "There is just not enough inventory; people need to fight over the few homes available on the market."

Historically, roughly 1.2 million new homes hit the market every year, he said, and so far this year, only 800,000 have been built. "It's been below that in prior years, and in the past decade greatly lower than that. Today's shortage is largely explained by a decade of underproduction."

In some hot housing markets, three weeks is an eternity.

"If we make it three weeks in our market, there is something wrong," said Darlene Umina, a real estate agent in the Boston area. "These days, you know within the first weekend whether the price was right." The median home price has shot up almost 9% in the past year to $561,300 in Boston, according to Zillow.

John Kasprzyk sold his home in Waltham, Massachusetts, in September in less than a week. The home hit the market on a Wednesday and he had an all-cash, inspection-free offer that was $41,000 above his asking price before the end of the weekend. It was one of 11 offers he received.

"We knew the market in that area was hot, but we didn't expect a cash offer to be this high," Kasprzyk said. "It was a big relief to walk away with equity."

He purchased the home five years ago for $455,000 and sold it for $630,000.

Across the country in San Francisco, one of the hottest markets in the country, real estate agent Erin Thomas has had buyers show up to an open house and hand her an offer. Offers well above the list price with no contingencies are common. "It can be very exhausting for buyers," she said.

To be successful, house hunters need to be prepared with a loan pre-approval letter, know their budget and home must-haves and be ready to dedicate their weekends and evening to touring homes -- and battling crowds.

Calculate: What will your mortgage payment be?

Earlier this year, Umina hosted an open house where she had to stand outside to greet potential buyers because it was so packed. She ended up with 18 offers on that home, three were all cash.

To help get their foot in the door, buyers in Denver have increasingly been adding escalation clauses to their offers, where buyers pledge to beat a competing offer up to a certain amount.

Denver's market has been on fire recently, with home prices jumping more than 7% in the last year.

Steve Thayer, owner of Keller Williams Action Realty in Denver, said he's sold more than a dozen homes in less that two weeks so far this year -- many of them going after one weekend. He's also gotten a few blind offers from buyers before they've even seen a home.

"One of the challenges in this market is getting to Saturday [for the open house], he said. "People are begging to see the house early and before it's ready to show."