There should be no limit.
You can't answer this question in any meaningful way in relation to E
In something like WoW for example, mobs, crafting etc are all balanced to where a given level of player should be playing. The zone is geared to their level and is somewhat of a challenge. It is neither lethal nor crazy easy. This is all done by creating the mobs to match the requirement. Density of mobs contributes to the difficulty, but as they (the density grow) so does the reward, or there will be a 'named' mob to give a decent reward.
In E

they add tougher ship to existing zones which is backwards. The missions in no way reflect the difficulty nor the reward reflect the time/effort invested.
The closest they came was with long distance Pax missions, which rewarded based on time invested. How-ever, it would be fair to say, they were a little too rewarding. But how do you measure that? You just cannot in E

because it too much based on Random Occurance.
And I fear, that in FDev they don't have anyone with 'balancing' experience. The recent skimmer missions taken off line, will be fixed using some obfuscated convoluted system that will never work, because, the mission system is also a mess.
There are a multitude of ways to easily fix this, rather than simply remove them entirely. But I'll be honest, I think the whole mission system needs a rework. I was doing Data Delivery Mission last weekend for fed rank and was earning more on time bonuses than I was on the actual missions. a 42k missions should not pay me and extra 120k for being early!!
For giggles; I feel I'm doing ok if I earn 10m per hour in the HazRez. But then again, I don't need credits, it's more a sense of reward vs effort.
At the end of the day, only the individual can assess if they feel they are being rewarded enough for what ever effort they put in. And if they don't feel rewarded, there's always other games with other micro transaction shops to feed your money to.