Hi Prabu:
https://community.marketo.com/MarketoArticle?id=kA050000000L5seDuration plus Must End On. The campaign first waits for a duration – a specific number of hours, days, weeks, etc. It then additionally waits until a specific day of the week and/or a specific time. For the day of the week to wait until, you can specify a range (Monday – Friday or Sunday – Thursday) or name a day of the week (Monday, Tuesday, etc., through Sunday). Suppose if a lead generated at 8AM Wednesday, will it receive the email from this campaign when it run at 9AM Feb 20 Wednesday?
My interpretation of the above would be that a lead filling the form in at 8am on Wednesday 20th would be sent the email on Wednesday 27th as it must wait the 1 day and then will remain in the wait until the following Wednesday. You may wish to run a test for reassurance.
An alternative would be to use a scheduled campagin and in your smart list do filled out form with date of activity = last 7 days. This will mean at 9am on Wednedays (your time as set in the schedule) Marketo will look for anyone who filled out the form within the last 7 days and then apply the flow.