Ok ok ok, not 100% "as long as you want," but the question really wasn't "Can I pay $40 a day for a mooring to leave my boat for a month and take the express back?" I inferred the question as "Can I go ashore for awhile and leave the boat?" Anywhere on the island it's fine to go ashore for quite awhile... Like the entire day... while the boat is on a mooring. The isthmus has this effect on me. We'll go ashore to hike, look for buffalo or foxes,etc. Well, we need to start the hike with a coupla buffalo milks cause.. well... we gotta walk by the bar anyway, may as well stop. Then maybe some hiking. Then some buffalo milks. May as well get something to eat. Something to eat goes well with a buffalo milk. Summer weekends there may be a band, so hang out to listen. Or chat with other boatie types. Lather, Rinse, Repeat. Get back to the boat by whenever.
I'd sure save a lot of $ if I didn't like buffalo milks so much. (For those not in socal: It's an alcoholic drink that has absolutely zero milk from a buffalo in it)
To the OP: It's an easy trip. You'll find somewhere to put your boat. Just go, you'll have a ball.