The hydragena will use 20 years to cover that fence.
One every 3 to 4 foot will be okay in 9 or 10 or so years.
To cover the fence I would go in a mixed direction.. some hydragenaes, and some clematis or honneysuckles- the lasts will do great the first years.
I managed to see one tiny flower on my hydragenae just before I sold my old house, at that time it have been planted 6 or 7 years and been moved once, with the root ball.
New one planted 2 years ago and I can not see any differnces.. darn it is so slow.