Any ballpark idea as to when it could get very out of balance ? I suppose lots of variables.
First...balance is just all the cells being at the same voltage at some particular state of charge (SoC). It can be a measure of how different the cell groups are in characteristics....but while you can fix balance by making all the voltages the same, you cannot fix the differences in characteristics--those only grow more different over time unless you replace all of the cells in the pack with matched-characteristic cells.
Whether your pack gets out of balance, and how long it will take, depends on what they built the pack out of. If it was one of the cheap packs, it's probably made of cheap non-matched cells with significantly-varying characteristics. That means that the groups made of those cells will probably also vary in characteristics (capacity, internal resistance, etc) from each other. That then means the charge and discharge process will not happen equally in every series group, and so the state of charge of each group will vary a bit on every cycle, adding up over time, unbalancing the cells.
What it really amounts to is that if they matched the cells (not a voltage thing despite what their sales people will try to tell you) then the pack will remain balanced without a BMS or anything else doing this for it because all the cells are the same and will all charge and discharge equally.
If they didn't match the cells, which is extremely likely, then the pack doesn't have the same capacity/etc in every cell or group of cells, so it can never remain balanced. A balancing BMS can make all the cells the same voltage at whatever SoC you've set it up to do that for (usually full charge), but it can't fix the underlying problem.
If you want to know if you will have a problem or not, measure all the cell groups when you get the pack, before you do anything at all with it--before it is charged or discharged, etc. Start a notebook with all the data by date/time/state of charge, etc., noting why each test was done, so later you can go thru it to troubleshoot if necessary.
If they are all exactly the same to at least hundredths of a volt, that's a good sign.
If they are not all identical, but many groups are the same as each other, then any groups that are different from the others are going to be problematic and grow worse over time.
If most or all of the groups are all different from each other, then the entire pack is going to be problematic, and grow worse over time.
If the pack isn't fully charged at this point, go ahead and do that. Then remeasure all the groups.
Once it's charged, do a discharge test on the bike/etc that it's intended to power, for as much of the pack's capacity as you can. Then remeasure all the groups.
Then recharge it to the same state of charge it started out as. (meaning, until whatever cell group you are monitoring voltage for during the recharge reaches the same voltage it was at when you did the initial measurements). Stop the charge there and remeasure all the groups.
If the cells have changed a lot in relative differences since the original test, they're not going to fare very well over time.
If they are identical in relative differences since the original test, it's a good sign that they are at least not going to get worse quickly.
That gives you a baseline to compare to as the pack ages over time, and lets you know what to expect from it.
