See the link below, from what I can see some paints become ineffective after 30 days of being out of the water, some don't. (see All you want to know about bottom paints)
The other link goes to an EPA sheet which in the following paragraph details the concerns with bottom paints.
"The highest concentrations of TBT and high
levels of copper have been recorded around
marinas where hundreds of recreational boats
are docked for long periods of time. Because
marinas are constructed where boats are
protected from wind and waves, the pesticide
concentrations are not as readily diluted by
natural mixing with adjacent waters. These
pesticides can accumulate in high
concentrations in the sediments and may have
a significant impact on the environment even
after the source of contamination is gone."
http://www.p2pays.org/ref/05/04853.pdf
Personally, I don't use a bottom paint. The bottom of my boat gets cleaned after haulout. I scrub with a soft brush on a pole once or twice during the season and the bottom is fine. Also the lake is better off without the added chemicals.
All you want to know about bottom paints