Retrogrades are a mixed batch --
Slow planet retrogrades are not too devastating, because they go on for long periods, we are used to them, and the planets themselves are more generalized in their effects than the fast planets. Fast planet retrogrades, especially Venus and Mercury, are more problematic. Mars is sometimes included with those, in part because of its ability to predict wars.
So, speaking very broadly, the retrogrades most astrologers fret about are Mercury (communications), Venus (love), and Mars (activity).
WHAT TYPE OF SPELL WORK?
Since a skull candle deals with the mind, working it during a Mercury retrograde would generally not be advantageous.
If it was being worked for love, attempting the spell during Venus retrograde would also be disadvantageous.
If it was being worked to drive someone into an angry fury, Mars retrograde would delay or slow the procedure.
So, you see, there is no one answer for that question.
TRANSITING ASPECTS OF THE RETROGRADE PLANET TO OTHER PLANETS
But it gets much deeper:
The retrograde planet may be making a "good" (positive, supportive) aspect to any planet in the sky during the retrograde, and that would make the retrograde milder.
Conversely, the retrograde planet may be making a "bad" (conflicting, opposing) aspect to any planet in the sky during the retrograde, and that would make the retrograde more difficult or more negative.
TRANSITING ASPECTS OF THE RETROGRADE PLANET TO THE NATIVE'S NATAL PLANETS AND HOUSES
In natal astrology (birth chart astrology) the person whose chart we are looking at is called the "native" (from nativity, the birth). Each time a retrograde occurs, it will or will not make "good" or "bad" aspects to the natal planet of everyone on earth, and in the case of a skull candle, the natal chart of the target should be looked at: how does the retrograde aspect the native's natal-position planets and in what houses (sectors of the sky) are the retrograde planet and the natal planets located?
For example: if the native's natal Saturn is in the 6th house (health) and it is opposed to natal Pluto in the 12th house (secrets), the person will be prone to having health issues and secret enemies and undiagnosed illnesses. Then if, in addition, Mercury retrograde stands square to the both of these planets from the 3rd house (communication), there will be delays in getting medical help and a health collapse may ensue. If your hope was to use the skull candle for a love spell at this time, it would be inadvisable. If your intention was to cause someone to suffer a relapse of a long-hidden mental illness and a delay in diagnosis and treatment due to lack of communication between medical personnel, this would be a fairly advantageous time to do your worst.
The subject of astrology is too complex for me to answer your general question with a simple "how it will be" conjecture.
However, you can make some determinations for yourself by seeking out an astrologer who does elections (finds the best times to work a spell) or by applying the principles i have outlined here for yourself.
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