Here’s a general rule for anybody starting their own business or freelancing: under-promise, overdeliver. If you think you can complete a task in a week, say it’ll take two. When you hand it in before the deadline, your client might be impressed with your timeliness and use your services again.
This will get you pretty far in any business you conduct. If you can surpass the expectations that you set up, you might be able to receive a long-term contract from your client. Just remember that you should stick to whatever deadline you set, so if you have to put it off for a few days, make sure you deliver on time at least.