The rule of thumb is to just test it on your chosen platform, with your workload. SSD performance matters a lot obviously, you'll want to run PCIe 5 if at all feasible, or a recent M5 Pro/Max/M6 Mac with high storage capacity (lower tiers have much worse performance). You can figure out a worst case easily (assuming that the fraction of sparse params you manage to cache in VRAM or system RAM has a fair chance of being picked among the total) but in practice you do quite a bit better than that - which is why even very small caches can be potentially worthwhile.

That sounds like not a rule of thumb tbh

The implication is “there isn’t one”.