Considering it's a part of the site's title, I wonder if 'practice' means the same to all of us.
And how do we judge when we've 'done enough practice'?
When we (or our parents, or teachers) decide that point has been reached, what criteria are used?
Is it when we can play something flawlessly, or when we can play it without the sheet in front of us, or when we can put our own expression into it? Only to not be able to do that the very next day?
Enough philosophy - although answers would be appreciated!
Of course, the parts we find most difficult are those which need more time and effort, so will need practising more. In isolation. Certainly not starting at the top again, and floundering on that same part. Just take that part, split hands, play slowly. 10 times? 100 times? Until it can be done blindfolded? Probably until it sounds as good as the easier bits.
Ah, the easier bits. When a difficult part is satisfactory, that's the time to join it back up with the surroundings. Maybe even start at the beginning, playing through smoothly until the end of that tricky bit. Depending on how far away a performance is scheduled, the practice regime will vary. With a week to go, all the hard parts should be sorted, and the up-to-now neglected easy bits brought off simmer and onto the boil with the rest of it. They won't have been forgotten, so a short burst of heat - practice - will have them up to scratch.
As to how often - it's subjective. For me, once a month. A concert pianist, the week before the next performance. Someone with little retention, every day.
Preparing a pieces for performance is like cleaning a window. If it's pretty clear, but there's a splodge in one corner, no point in cleaning it all again - just the mucky bit. If Great Aunt is visiting next week, keep an eye on it, but do a full clean not long before she arrives.
You've answered your own question, in that if it takes 30 mins to re-kindle the easy bits, after neglecting them while you sort out the others, the question is now 'at what point do I find and use that extra 30 mins to polish the easy bits?'