I have heard some writers be so disciplined that they would plan to write for an hour, each day, from 7am to 8am, without fail.
Others will write with caffeine enhanced super abilities, and will go on an eight hour marathon from 11pm to 7am the next day.
Other notable writers, like Stephen King, have been quoted as saying 'I used to write 3,000 words a day', now it is more like 1,800.'
You know what? All methods are fine, because they work for that particular writer.
There is no one best fit, only the one that is most suitable for you.
For my part, I write when I can. If I am really 'in the zone', nothing and no-one can shift me.
But generally, I will write as much as I can. Even if it is a single paragraph, so long as it is worthy of the the manuscript as a whole, then I have had a successful writing day.
I think the other key thing is not to beat yourself up on missing writing targets. Why? Because writing should be a joy, not a chore. I don't deny it is hard sometimes, but if writing ever becomes a chore I would give it up and do something else with my time.
I've always stated that writing was very much a side project for me, and it is possible my writing is more amateur as a result. I have no doubt others write with better prose, style and depth than I ever could.
But through practise, I am learning, and will continue to learn.
Another point of note is the type of writing you are doing, If it is non-fiction, actually I find that harder, even though you tend to have done your research and have the facts in front of you.
Fiction can often flow much easier and you can always delete the 'what the hell did I write there' stuff at a later date.
Write. Enjoy it. Never let it be a chore.
With that, I'll wish you a great week of happy and successful writing!