There is no strict definition, because a crappy sailor can do coastal on a seaworthy boat, and a really good sailor can take an inappropriate or marginal oceanic boat over the horizon.
I think you can say 24 hours x 5 knots=about 120 miles of range under sail. Most boats in the 25-30 foot range will not have enough fuel
to do that without a jerrycan or three, but some will. Others will be able to handle heavy weather, while some will be light enough to make distance with little wind.
I think that the expectation
of a coastal boat is usually "fun, easily driven, good for having a beer on at the end of a day". 99% of the time, a production boat for coastal cruising will cover this off really well, as most people "coastal cruising" favour fair weather and will motor under 10 knots and head for shore over 20 or four feet of waves.
So if you intend to "keep on keeping on" in the coastal mode of being out of sight of land, but within a day's sail (like going from Florida to the Bahamas, say, or Maine to Nova Scotia), you'll want a somewhat different boat and a more robust skill set than a club racing Catalina 30 owner who will look for three days of 12-18 knot winds and two foot waves before contemplating a point-to-point trip down Lake Ontario, for instance.
A "coastal" boat can expect to encounter bad weather, but of short duration and maybe medium severity. Summer squalls, for instance, can be fierce, but don't typically develop the huge and long waves of the open ocean...the fetch is too short.
This is why it's a difficult question; when people say "coastal", I assume they mean "daysailer or fair-weather cruising". There's a lot of boats up to that, but which I would hesitate to put out of sight of land or in 35-40 knots, because they might prove too tender, too light, too ill-equipped for reefing down and too lightly built in the portlight, hatch
and companionway departments.