In theory $34.9K is the maximum safe from Proof of Income, not $25K that is reported in much of internet. Anywhere from $30-34.9 would be safe and what would i recommend the most. Crossing $35,000 is a little more tricky as accounts can be stopped at $34,900 and some others are like given the green light where once they have hit the magic number of $35K as long as you keep asking the system on $5K increments it will never ask you for any forms. On myfico.com forums some users have reported following this technique and grown the card up to $60K on $5K increases.
Being asked the form is not the same as FR. FR is something different, but for us that don't live in the USA i think its better not to even ever get the 4605-T form that is harmless, but could increase chances of FR if asked and not provided.
When Amex asks you this 4605-T form that is not much than a Proof of Income, it is optional to provide. You can just ignore it and continue asking for credit limit increases etc and nothing has ever happened to anybody, but i prefer not to ever even see a hint of this form.
It is kind of tricky to say if an account should push past $35K.
For example if i were to go from $30K --> $40K and i was approved, i am not sure if future subsequent credit limit increases will be free of a Proof of Income.
What in the forums only others have successfully tried is level the account at $35K on the dot and then keep 5 on 5 increments and that should work for a long time in the long run. I don't know for example would i had $40K on the dot and ask 5 in 5 increments if it would work as nobody has tried it. It sounds silly, and i could be wrong but from the data points i have seen... it could look like the account has to "touch" $35,000 on the dot.. that number so like then the Amex system continues giving credit limit increases without spitting the Proof of Income form verbiage.
Idk, there are so many variables... I'm just replicating the same limits progression as the person who was successful in bringing it to 60K. By chance my card and his started the same: 10K--->30K--->35K.
