In case I write IELTS again after submitting my application for Express Visa, can I withdraw previous application and submit again? Also, how long should one wait before being informed that his/her application is not picked?
There is no need for you to withdraw your application. Just update your current one and resubmit it.
You can just update your current application with the new TRF number for the latest IELTS.
If your profile is selected in Express Entry draw, you should ideally get an email and a message in your EE account within 24 hours. If you don’t get it, you have most probably not made through the draw.