Why would this be different to any other part of their job?
That means two things:
1) You don't get paid extra for it, it's part of what you do in the same way fixing bugs, talking to customers and everything else is
2) As it's part of your job you get time, support and resources to do it properly. If I expect you to produce a 500 word piece every month, I (as the employer) should give you the time to write it, to check your facts and put it together to a professional standard, and I should make sure that it's clearly prioritised in with the other things I expect from you.
If it doesn't fit into your regular working day then it should either be seen as overtime (and paid exactly the same as any other bit of overtime) or dropped as something which isn't realistic.
But broadly anything you do for the company should be treated as what it is - part of your job - and be handled in that way.
After all, if I asked you to present the companies product to a client and that wasn't part of your normal role, I wouldn't expect you to ask for more or different remuneration and I wouldn't expect you to do it in your own time.
Why would this be different?