it would take you roughly the same amount of time, probably more depending on the school. Masters courses are tailored for the students that dont have to worry about schedule conflicts with other undergrad classes. 

Sometimes theory and structural classes are taken with the undergrads anyway, so i see the benefit, but I havent heard of anyone applying to undergrad after getting rejected from the masters program, im sure someones done it. what school is this for?

the masters students where i went to school were older, and the only older person we had in our undergrad class, had no degree at all, just construction experience

I have heard of it being done at UT for sure, and I believe I heard someone else do this at another top school but can't remember which one.  The main difference in admission is that you don't have to submit GRE's or Portfolio (or so it seems.) I have everything done but most of the portfolio to go. That said,  I have though about this given my relative lack of knowledge in the field of architecture.