Well smart people here will tell you indeed that the time to buy LEAPS is when IV gets crushed. I think they have some tools that indicates this, maybe their broker show it on their options-chains, I don't know, but I have no idea where to see it. However, as I have a range of calls in my trading account, and I check it regularly, I can see the changes reflected in the % up/down and yes, right now LEAPS are down, so a reasonably good time to buy.
Otherwise I'm just using feeling/common-sense/belief, call it what you will. Example: In early March, just before C19 struck, the SP was trading around $750 having pulled-back from the February highs. Seemed to me that the stock wanted to go higher, so I bought 4x Jan 21 $875's for $14k each, total cost $56k. Of course then we got the C19 dip and their value was massacred down to 15k, did I panic and sell, hell no, obviously it was going to rebound at some point and having 10 months still to run is the beauty of LEAPS. In fact close to the bottom of the C19 dip I sold my 100 trading shares at $465 and bought 10x Jun 2022 $1400's for $44k total cost - sold those 3 weeks later for $120k, when the SP had risen to $700ish and then started selling puts with the capital, netting $5-$10k per week for a while.
As for the Jan 21 $875's - they went ITM in June and I sold for $108k, so roughly double the money, waited an hour for a dip, then bought 4x June 22 $1250's for the same premium (know as rolling), thus dramatically reducing the risk. Obviously those calls are now ITM and the current Ask/Bid is around $650, so they're worth nearly 3x (so 6x original investment overall).
If the SP goes $4k or above before June 2022, those 4 calls (original price $56k) will be worth $1m+, roughly 3.5x more than the appreciation of common stock had I bought that instead.
So yeah, buy LEAPS on the big dips, basically, then roll them to less-risky strikes/dates. Not advice, of course, always a chance you'll lose your premium.
As for the 10x Jun 22 $1400's, well I should have held onto those, they're now worth $575k, and this is my second biggest take-away with LEAPS, don't sell too soon, of if you must, just sell half, because they appreciate like crazy...