The building could be for sale for 10 years and the seller may have paid a dollar for it that still doen't mean they are willing to give it away. prepare a written offer if it isn't accepted modify your offer and look towards another investment.
Realtors are there to be bothered thats our job to provide dervice use your agent.
I'm really interested in finding out why that is so important for you to know, as you browse homes on the internet, how long it has been for sale? Many buyers tend to really be interested in the Days on Market for the homes they are searching for because they have this false belief that these are the only homes in which they may be dealing with a modivated seller. The reality is, there is no immdeiate connection to the two and most likely that's why we don't see this information on websites. More important features to look at and note are price, style, location and square footage.
In this market it's common place to see homes sitting much longer for two reasons. 1) Because the inventory in most places out weighs the amount of qualified buyers and 2) Sellers were unrealistic with the state of the market when they first listed their home and have become more realistic since. Your doing yourself a grand diservice by just focusing on Days on the Market when looking for a home.
However, if your interested enough in a home to want to see it, then a call to your realtor should be fine.
A drawback to that site, and to others, is that if a property has been listed, withdrawn (or expired), then relisted, the new listing only shows the new DOM. So a property might have been on the market for 180 days, expired, then a few weeks later listed with someone else. You pull up that listing, and it shows 30 days on market, not 210...which is what you're really looking for.
There's no other way that I can think of, other than calling the listing agent for the property. And while that will spare your agent from the "bother" (as other comments said, most agents won't consider an inquiry from a client a bother), it won't save you any time or effort.
Hope that helps.