I was floored with glandular fever about 5 years ago. Was off work (without pay) for a month. Should probably have gone to hospital, truth be told - but stupid male in their 20s, 'tis nothing but a flesh wound.
It took me around 18 months to start feeling remotely like myself again in terms of work output and mental alertness. I am not trying to belittle long covid but I remember my Doctor at the time telling me something like that for each day you go back to work before you should, you will likely add at least a week to your long term recovery. I listened to a degree, but I just couldn't sit at home and do nothing any more, and that cost me as well.