You want to use them whenever you hit a new, BIG threshold of the VIP system.

For example, if you're VIP lvl 8, which only gives a 60% boost to Gold and XP gains while AFK, you shouldn't use any of the "x hours" items and wait until you hit VIP lvl 9, because at lvl 9 you get a 90% increase in Gold and XP gains while AFK-ing, which is a net 50% gain at that point.

Similar case when going from VIP lvl 5 to 6 (more than a 50% increase) and lvl 11 to 12 (about a 45% increase) and so on.

Generally don't use "x hours" items until you reach VIP lvl 6, 9, 12 and finally 15.

However, as soon as you do hit one of those thresholds, feel free to use some of them. For example, you've just started out and want to save all your "x hours" items until you hit VIP lvl 6, then wham, pop them all. Somewhere around lvl 7 or 8 VIP, start saving them again until you hit VIP lvl 9 and so on.