I bought my 33 year old boat 6 months ago. It had spent 2 years mostly covered and on the hard. (mostly, because part of the cover had torn starboard of the companionway, and the PO had cut an opening over the transom to get in and out.)
When I took ownership, the bilge had maybe 10 gallons of water in it. I shop-vacced it out and came back a week later (Regretfully, the boat was stored far from home and I could only visit on weekends to prep it for splashing) the next week it had another 10 or so gallons in it. Repeated that process pretty much every week until I finally put her in the water.
I'm still hunting down all the leaks, but one thing my slip-neighbor taught me is that a little tiny spider-crack in the deck can act like a straw and siphon down a steady trickle of water to the interior. My old girl has literally dozens of locations totaling hundreds of spider cracks. But on top of that, every single evening that the boat was on the hard, water condensed due to the weather, and then trickled into the various voids and pockets throughout the boat. The rear cabin wood was completely saturated and ruined (mostly the bed frame) and I'm fairly certain that the space between the deck and the ceiling in the rear was not cored, but was really good at holding water. My scuppers had long been clogged with leaves from the boat hauler lot, and so whenever it rained, the rear deck would fill up with at least a few inches of stagnant water. I think it seeped in around the pedestal and stayed between the layers of glass.
Fast forward to now, 6 months later. I think being on the water has helped tremendously. Also I spent winter as a liveaboard under plastic with a dehumidifier running keeping it pretty dry (the saturated wood dried out too). I think just the wave motion from being at the slip through your average windy winter was enough to let all that trapped water finally find its way to the bilge.
I've got my agenda to re-bed all my deck hardware, and for now I've been finding all the spider cracks and using Captain Tolley's penetrating glue to seal them up. (Long term goal will be to sand down and repair the gelcoat, but I needed a quick-fix). Most particularly I have been using the penetrating glue along my gunwales. There's zero chance of me taking those rails off to re-bed them, so I don't mind the glue there. And the PO looked like maybe he'd tried some kind of silicone or other to stop water from seeping in under the rail.
The water in my bilge hasn't budged much lately, even after heavy rain. I do still have the occasional gremlin (really need to re-bed the pedestal, but I'm afraid once I start that I'll need to replace the whole thing) but the overall amount of water intrusion is waaay down.
I guess the "TL;DR" is that a boat that spends a long time on the hard is going to have water in all kinds of uncomfortable places and will take months to dry out.
