There was a time when the standard contracts for billboards were 2 years in length. The advance payment was equivalent to six months rental applicable to the last 6 months of the contract. Then as years went by, the contract length became one year. Standard DP was still 6 months.
During my stint as OOH Media Strategist of UniversalMcCann, we were able to bend the norms more by asking for three month tactical placements. That was around 2002-2003.
Now, 2010, Three months booking has become the norm. However, this short term placement does not come without a price to us who are on the agency and advertiser's side. The downside of the three month (or one month placements) are:
-Minimal room for nego on cost
-Payment due to supplier before posting
-No more right of first refusal or first option to renew
I guess the last part is very understandable because 3 months is now a very short time to nego for the next incoming client. With escalating costs to operate a billboard on supplier's side, suppliers need to pre-sell the space in order to minimize the billboard's downtime.