In reply to llanberis36:
There are many takes on it, some reserve at last hour, some do it well in advance.
As for all the huts, even if you reserve in advance, they always ask you to confirm your arrival a few days prior. So, normally in the last 7 days before your selected date you will see some movement and cancellations being done - which probably means you can get a place without issues.
That said, in most cases reservations are reimbursable (if they charge any money upfront at all when booking, ie Gouter hut) if cancelled with sufficient time (normally 3-4 days), sometimes you may be charged a small admin fee. With that in mind I prefer to reserve in advance, even few days in a row and then, looking at the 5 day forecast, modify the reservations as I see fit. But I am a guy that definitely likes to feel prepared for the trip and I wouldn't sleep well knowing I still don't have the bookings made two months in advance
Cheers!
RS