Please take a look at the 100+ reviews that I have here on Freelancer. The bulk of these are for auto accept projects which are very similar to your requirement. First need to login to the website, then monitor the site for an offer to become available and the to accept it. Many of my solutions are being used on a daily basis several years after they were written.
I cannot tell you how long this will take until I have had a chance to the see the website.
The basics, login and refresh can be done very quickly, but in order to determine what's involved to automate the remaining steps I need access. Some sites are very easy, others, especially those that use bot counter measures are more difficult.
If the site has plenty of offers, then things usually go quickly, but if the offers are only made available infrequently, then it could take some time to work through the whole process.
If you would like to discuss your project in more detail, I look forward to your reply.