These are the factors to consider when deciding the timing of buying a 24-hour collider charge:
Reasons to not wait:
Level thresholds: You expect the xp thresholds for levels you haven't reached yet to go up in the future.
Cash and time: You have a large amount of cash on hand and enough time to convert a sufficiently large amount to xp (sufficiently large means enough to earn much more than 5 donuts; I would say at least 100 donuts).
Various limited-time factors: There's a limited-time item or deal you want but you don't have enough donuts to buy it; or there's a limited-time item that gives a better return than rat trap trucks; or there's a sale on collider charges; etc. If you're about to get a large amount of xp from an unusual event (like completing a large building), that always helps, but it's a much lower priority than the other factors.
Reasons to wait:
Level thresholds: You expect the xp thresholds for levels you haven't reached yet to go down in the future. Based on the new model they've implemented where the thresholds no longer go down with level releases (and the fact that they already increased the scale on the new model once), you can probably have high confidence that this will never happen.
Cash and time: You don't have enough cash on hand and/or enough time to convert it to get a good return during the 24 hours the charge lasts.
Bonus percentage: Your current bonus percentage is too low to make it mathematically possible to earn more than 5 donuts worth of xp, or you have specific expectations that your bonus percentage will increase significantly. A higher percentage will provide a better return.
Need for donuts: You don't plan on spending any donuts right now, so it doesn't matter to you whether you get them now or later.
Various limited-time factors: You have reason to expect any of the various limited-time factors to occur at a later date that's worth waiting for. For example, you know a character will be released in the next act of an event, so you save up a little more cash while waiting a week or two for it to come out.
The reason your current level isn't a factor is because in the long run, if the listed factors don't change, then the total xp you will have gained will be the same regardless of where you position the gain from the collider charge within that long run. If you'll get, for example, 50 million xp from a charge, whether you get that 50 million today or a month from now won't change the total xp you'll have earned (and level you'll be, and donuts you'll have gotten from getting there) three or six or twelve months from now.
