You can get multiple sets by working longer, or one from a long rest
The feat says:
With one hour of work or when you finish a long rest, you can cook a number of treats equal to your proficiency bonus
So, you can cook one set for every hour of work, or without spending any extra work time, at the end of your long rest. In case of the long rest, you always get one set at the end of the long rest, never more.
Otherwise, it would have to say "for each hour of work, or for each hour of work during a long rest". And doing anything strenuous, which work falls under, is not "light activity" and would disrupt the rest.
If the rest is interrupted by a period of strenuous activity - at least 1 hour of walking, fighting, casting spells, or similar adventuring activity - the characters must begin the rest again to gain any benefit from it.
That's not what happens here, you somehow get it at the end of the long rest, and you can narratively explain this by saying you prepared it leasurely in the hours you were not sleeping, but in the long rest case, you get nothing at the end of an hour as you are not working, you are resting and doing only light activity.
The game in the crafting rules typically allows for 8 hours of work per day, so you could prepare such treats eight times a day, or more often if your DM allows.
Note that temporary hit points are not additive - the eaters would only always get the highest amount of temporary hp at any one time (here it's always the same), but they could benefit from eating another one, after they lost the temp hp somehow.
