Exterior paint on timber is not decoration, it is the only thing keeping water out of the wood. When it fails the timber starts taking up moisture, and repainting over timber that has already gone soft is covering the problem rather than fixing it.
When it needs doing
Look in spring, when a winter's worth of damage is visible and there is a season ahead to work in. Flaking, cracking along the grain, a chalky surface that marks your hand, or any bare timber showing means it is due. Paint that is merely dull and dirty is doing its job and wants washing, not redoing.
The places that fail first are the ones that stay wet longest: the bottom rail of a window, the lower edge of a door, sills, and anywhere two pieces of timber meet. Check those and you will know the condition of the rest.
Why it failed early
Paint applied over damp timber will not last, however good it is. The same goes for paint applied in the wrong conditions: too cold, in direct heat, or late in the day when dew will settle on it before it has set. A single coat over bare wood with no primer looks fine for one season and then goes.
Blistering with bare wood underneath means moisture pushed out from behind, so there is a water source to find first. Blistering with paint still underneath is usually a bonding problem on the previous coat.
Preparation that actually matters
Scrape back every loose edge until what remains is firmly stuck. Feather the edges so the repair does not telegraph through. Prime all bare timber, and prime end-grain twice because it drinks far more than a face does. Fill splits with an exterior filler rather than an interior one, since interior filler will fail in a season of frost.
Check the moisture content of the timber, or at least give it a run of genuinely dry days first. Painting a south-facing elevation in the morning and a west-facing one in the afternoon avoids working in full sun, which skins the surface before the coat has flowed out.
Rot: fill or replace
Small soft patches can be cut out and filled with a two-part wood repair product, and that is a sound permanent repair. Rot that is soft to a depth of more than roughly a centimetre, or that runs along a joint, means the section wants replacing. Filling it buys a year and no more.
While the ladder is out
Look at the putty or beading around the glass, and at the small drainage holes in the bottom of a window frame. Both are usually ignored and both let water into the frame when they fail, which is often the reason the paint went in the first place.