How long for? And how quickly? That will depend on the info!
7-7 is within acceptable limits, any longer would be a bit much.
Before you start looking note down what you want your nanny to do, how long you're likely to need them for and how much you're willing to pay. Are you so incapacitated that you need someone to take over totally (therefore very experienced and capable) or are you able to supervise (therefore might suit someone less experienced and definitely someone used to working shared charge)? Do you need them to do household duties?
Agencies, assuming London, with good (and efficient temp desks): Eden and Imperial. If you can narrow down location a bit more there may be more local agencies who can help you. There will probably also be nannies in your local area advertsiign on gumtree of netmums but if you're out of action you may not feel up to doing a 1st time nanny hunt solo.
Check their qualifications, CRB and reference even if from an agency - they should be able to show you these at interview and provide you with phone numbers for referees. For temp nannies don't be put off by a CV with lots of jobs as if they've temped professionally for even a few months they could have had 10 jobs. Aim to check the last 3 references.
It's probably better to get someone who is accustomed to temping continuously. Firstly they won't leave you for another job, secondly they're used to working in all kinds of situations so will be pretty flexible and thirdly they may meet the criteria for self-employment and have all that set up which simplifies your life considerably.
If you need a nanny for an extended period of time, however, you may need to become an employer in which case outsourcing all that to a payroll company, such as Nannytax or PAYE for nannies is probably the simplest (if not the cheapest) solution.