This is a toughie as my version would be pretty far from anything that would pass in the current world anyway.
The first seemingly obvious restriction would be a minimum age. I would imagine even some of the most staunch pro-choice peeps would agree there has to be a minimum level of emotional maturity involved for this to be considered a reasonable decision for someone. I would also say there should probably be a medical waiver for the limit in certain cases since some young people do get terminal illnesses that would needlessly subject them to daily suffering till nature steps in.
Once you establish an age limit, then the restrictions would be relative to the social safety net in the country in question. For example, here in the USA it's been made pretty clear you have no value to society unless you can come up with some way to extract profit or be a cog in the process of creating profit for someone else. In this situation where there is no guarantee of basic survival, there should be no restrictions beyond age. This would be both merciful to the "unprofitable" and just good business as the cost of some N and the medical personnel time to administer it would be minuscule compared to the costs that will come with societies ineffective attempt to act like they care when it's pretty clear they are just going through the motions.
I can't speak of other countries and how serious they are about protecting the "unprofitable" but say in a place like Canada where there is at least a guarantee of basic medical care, perhaps some additional barriers would be appropriate. However, if a nation is not prepared to guarantee at least food, shelter, and basic health care as human rights, then one should be able to opt out whenever they reach the minimum age and decide life centered around materialism/profit is not for them as we can't all be greed obsessed sociopaths.
Obviously, the reality is there will be an insane amount of restrictions if such a thing even manages to come to pass to begin with, but given enough time I feel we will eventually reach the "suicide booth" option. We will all be long gone before we get there unfortunately.