First of all you really need to think about a hook to get people's attention. You cannot just try business as usual. After all you audience did not hear for you for some time and will therefore not be:
a.) looking out for your emails anymore and
b.) not knowing the sender or ignoring it

So, try a funny angle, a mea culpa angle, "a you would not believe angle", etc.. Anything that gets at least attention and whatever may fit to your audience. Also make sure to run an A/B test for the headlines. If you are using an automated e-marketing platform such as Active Campaign or similar, they have an in-built functionality for that.

Now to the second part of your question. There is no hard number for how long should you send emails until you delete someone from your list. But my consideration would be the following: You have once spent most likely a significant amount of time and money to create that list so I would not give up any member easily until it is clear he is definitely not interested anymore. Of course defining the last part is not easy. But I would consider several e-mail, over several weeks and then follow up with an e-mail explicitly asking do you still want to hear from me and an opt-out and reactivate link.

Hope that helps. Best of luck.

Gerd TF