~~Same as anyone else really. "Professional" here just means to treat it seriously, be ready to sacrifice, devote fully to the cause. This phrase never meant a career path within the bourgeois society.~~

I was talking out of my arse! Went back and found the relevant place in Lenin's "What is to be done", chapter 4 part D, completely missed or forgot it:

https://www.marxists.org/archive/lenin/works/1901/witbd/iv.htm

To be fully prepared for his task, the worker-revolutionary must likewise become a professional revolutionary. Hence B-v is wrong in saying that since the worker spends eleven and a half hours in the factory, the brunt of all other revolutionary functions (apart from agitation) “must necessarily fall mainly upon the shoulders of an extremely small force of intellectuals”. But this condition does not obtain out of sheer “necessity”. It obtains because we are backward, because we do not recognise our duty to assist every capable worker to become a professional agitator, organiser, propagandist, literature distributor, etc., etc. In this respect, we waste our strength in a positively shameful manner; we lack the ability to husband that which should be tended and reared with special care. [...] A worker-agitator who is at all gifted and “promising” must not be left to work eleven hours a day in a factory. We must arrange that he be maintained by the Party; that he may go underground in good time; that he change the place of his activity, if he is to enlarge his experience, widen his outlook, and be able to hold out for at least a few years in the struggle against the gendarmes. As the spontaneous rise of their movement becomes broader and deeper, the working-class masses promote from their ranks not only an increasing number of talented agitators, but also talented organisers, propagandists, and “practical workers” in the best sense of the term (of whom there are so few among our intellectuals who, for the most part, in the Russian manner, are somewhat careless and sluggish in their habits). When we have forces of specially trained worker-revolutionaries who have gone through extensive preparation (and, of course, revolutionaries “of all arms of the service”), no political police in the world will then be able to contend with them, for these forces, boundlessly devoted to the revolution, will enjoy the boundless confidence of the widest masses of the workers. We are directly to blame for doing too little to “stimulate” the workers to take this path, common to them and to the “intellectuals”, of professional revolutionary training, and for all too often dragging them back by our silly speeches about what is “accessible” to the masses of the workers, to the “average workers”, etc.