Time-frame; forever. Or until he closes the business.
Strategy:
Step 1: Assessment and analysis - check the current situation with the site - traffic, stats, serp overview, backlinks, content, pages, keywords, competition, on-page status etc. Document everything so you can refer back when assessing progress.
Step 2: Planning - come up with a plan about how you are going to proceed. Look at how many pages / products they have and put a plan in place for doing keyword research for each page/product. Look at on-page changes that are going to be required, look at the links you can build based on what you suggested above as well as what the competition are doing. If its a big ecommerce site you could probably plan in a years worth of work or so at this stage.
Step 3: Start doing the work - improving on-page, copying competitors backlinks, building new backlinks etc.
Step 4: Assessment - assess the progress every month in terms of traffic, stats, serp overview, backlinks, content, pages, keywords, competition, on-page status etc. To begin with this is going to mainly involve producing a detail of the work you have done and as time goes on its going to be showing improvement in site stats, then improvement in keyword visibility, then ranking, then traffic, then sales.
I would say to them give a 3-6 month window in which you will see work done + slow improvement in stats, keyword visibility and then in 6-12 months you should start to see month on month improvement in serps, traffic and sales. Thats a bit of a guess as I don't know what you are going to do or what you are going to achieve, but I'd say thats a fair proposal to make to your client.
As for the actual strategy that you employ to get these results - like I said, its going to be all about researching as much as possible to get good and sensible keywords to target for each product/page, then optimizing the pages for those keywords, making sure on-site structure is good and at the same time slowly and steadily building strong backlinks to the sites homepage, category pages and product pages.