Its a tricky question. Because ppl tend to tell only part of the truth to look like they are better, than other players. Server usually updates your wallet each 20 minutes. Some players call it a ‘tick’. The bigger is the difference in your wallet after update, the more effective your way to ISK is.
Let me give some examples: normal ratting in null (plain orbiting rats in Vexor navy Issue with oversized prop mode ) will upgrade the wallet content somewhere in between 15 and 20 milions per tick. So - 45-60 milions per hour unless you start looting / salvaging. But looting and salvaging takes time.
Ratting in carrier can give you more (people boasting about 250 milions per hour) but there is a trick - you need special type of sides, you will pay for fuel and lost fighters, your time between press ‘undock’ and start appling damage is … large. So reality is, that ‘null 1337 ratter’, who payed with real cash to get into carrier (skill injector and ship itself), will shoot the site , that will give him 30 milions, in 10 minutes. That means 30 * 6 = 180 milions per hour. Awesome. Now other 5 minutes to align/warp/dock/undock/scope fighters etc… Plus strontium calrates or whatever fuel they need to make that thing move … Plus another 10 minutes wait till more sites like that become available. So you are back on 80-100 milions per hour. If you are good. But he will still boast about 180 milions per hour to justify that he spent real money on game, that can be played for free (I do not mean suscription - its another story).
Very similar it is with L4 security mission blitzers, who rush through couple of very effective missions , that gave them good money, but then spend hours to get their standings back. That rush they count, hours to grind standings are not counted.
In reality you are good if you make 100 milions Isk per hour with all trips / dodging gankers etc…
It can be (hardly) achieved in high sec (L4 missions, incursions) , pretty simple in null sec (after you get your carrier right and find empty space where all rats are yours) and very simple in WH (C3 and higher, but you will be ambushed a lot). Null and WH space ratting are ticket to pod way too often, low sec is not for making ISK - it is for PvP fun. Exploration is a gamble. You fly months for low profit, than you get jackpot and make a bilion in 5 minutes.
Only solid source of stable high income is trading, but for me it is boring as nothing else and IMHO not proper way how to play this pew - pew game. But totaly legit and very effective, so if you are not against playing “excel sheets online” instead of “eve online” - feel free to enjoy 0.01 ISK PvP in trade hubs.