None
In answer to your question, the answer is no amount of vapor is 'safe'. Every vape you take,,,from the very first to the one you just exhaled, no vape is safe.
YOU ARE SUCKING IN HOTTER THAN NORMAL VAPORS,,,WATER, FLAVORING, PG, VG, POSSIBLY NICOTINE
That is NOT what was meant to go into our lunges,
Vapor is better than smoking but you asked 'safe',,,,well, it's stupid to inhale the stuff we do, intentionally, when the verdict is till out and it is still questionable regarding the effects of nicotine and VG and PG
My opinion is, however, that the amount that is prudent would be the amount you need to satisfy either the nicotine addiction or the habit of smoking addiction, (fingers, hands, lips, mouth, warmth, sting, taste)).
I do morning needs vaping a moderate to high nicotine juice (18 to 24 mg)
I will drop to 12 mg juice
Then the early evening to bedtime will be reserved for primarily zero nic, maybe something higher when I go to bed.
I suggest vaping what feels comfortable to you but that has a high enough nicotine content to give you both the TH and 'rush' you experienced with analogs,
Vape what it takes, but realize and try to find, the lowest nic,,,even if it varies during the day, that you need to make you feel 'normal' and satisfied,
Do no more and no less and you will be satisfied,,,doing the best and safest for your body at that moment, and doing what I would consider 'prudent', or if you insist, 'safe' for you,
Enjoy