A rough guess for me would be around every 20-30 mins, I created 2 accounts to test this and starting at nastya at level 1 with (x100) modifier and went over Fort which took 15 mins (Killed everything along the way) I recieved 1 mil according to my other account looking at my stats along the way, then killed multiple bosses and recieved around 2-3 mil right after I went into outpost "'Note' it took me 20-25 mins to kill the bosses." All together I reached level 40+ in less than an hour and was still able to gain xp for about anouther hour doing the same trick of getting 2-3 mil up until I met and killed a Flaming Black Titan which gave me too much experience.
The next day after the 24 hour ban I went to Secronom Bunker and grinded roughly 1-2 mil in roughly the same time interval (20-25 mins) and was perfectly fine. But as soon as I fought any Flaming boss and recieved their exp along with the extras I killed and went to outpost, I recieved nothing. So my suggestion would be to stay at Fort Pastor with the Exp mod if you want to farm bosses and be a little safer and just keep an eye on your exp. *Side note, I'm not certain about this but it has happened everytime I've used the Xp hack both x4 or x100, but during outpost attack at either Fort or Secronom you will get 24 banned almost everytime if it is active, so just be weary of that, I've tested at other outpost during outpost attacks and it works fine as long as you go in every 5 mins.
Hope this helps prevent more 24 hour bans for the Exp Mod.