Calmac publish timetables which will tell you how long the ferries take.
While generally it is advised to travel South to North due to prevailing wind that cannot be taken for granted and I remember sailing south meeting cyclists grinding northwards.
Clisham from Harris to Lewis is a beast of a hill but there is nothing else of any significance.
The southern islands are fairly liberal re the sabbath but Lewis is predominantly shut tho' nowadays that is changing.
It is as well to remember that many shops and even B&B places will be closed on Sundays tho' ferries still run nowadays.
I recall having to pedal downhill quite hard at times against the wind and that was going north..
The wind seems to come from all directions at once.
But there were beautiful still sunny days too
How long does the trip take..
Depends on your style..
(And the wind,)
I think I spent seven nights from bottom to top on the last but one visit..
But I get hopelessly distracted by side roads, nice beaches and other interesting diversions.
As well as slowed by headwinds.
I think the last time I went I meant to stay in some of the historic 'black house' hostels , but never quite got the timing right, and ended up camping every night.
Given that you can camp just about anywhere it seems a shame not to give yourself that flexibility, and take a tent..
One night I even ended up camping on a golf course..
Nobody seemed to mind (or notice..)
