Neil, the direct answer on timing: for a hoist I would start looking 8 to 12 weeks out and have it confirmed no later than about 4 weeks before you fly, and for a shower chair 2 to 3 weeks is usually plenty. The gap between those two numbers is not fussiness, it is stock. Shower chairs and commodes are commodity items that most hire companies hold in quantity. Hoists are not: a company may hold only a few in a given area, they are seasonal, and in a busy resort in high summer they go early.
What it depends on, and this is the part that catches people, is that a hoist is not one item. You need the hoist itself, and a sling, and the sling has to be the right type and size for your wife, and slings and hoists from different makers are not always compatible. So when you ring, do not ask for a hoist, ask what model of hoist, what safe working load, what sling types they supply, and what sizes. Take a photograph of the sling she uses at home and the label on it, because the label carries the model and size and that one photo answers half their questions. Ask about the charger and the battery too if it is powered, and whether they leave a spare battery, since a flat hoist on a Sunday is a genuinely stuck situation.
Practical points on top of Sue's list, which is a good one. Ask whether they demonstrate on delivery or just leave the equipment, because an unfamiliar hoist and an unfamiliar sling is exactly where injuries happen, to the carer as much as the person being lifted. Ask what the callout is for a fault, and get a phone number that is answered outside office hours. And confirm the collection date in writing, because being charged for an extra week after you have flown home is a common and annoying way to end a holiday. The whole hire process, including what to check on arrival, is in hiring mobility equipment abroad, and the room measurements that decide whether a mobile hoist can even be used are in accessible accommodation.
One caution. A mobile hoist needs clearance under the bed for the legs and floor space to turn, and a rented apartment with a divan base or a bed against a wall can defeat it entirely, so ask the owner for a photo under the bed and of the space beside it. And the part that is not mine to answer from here: which hoist and which sling are appropriate for your wife, and whether you are safe to hoist her single handed in an unfamiliar room, are questions for the OT or therapist who knows her transfers. Please take the specifics to them before you commit to the booking rather than after.