Welcome to the forums.
Realistically, as comfisherman states, there are lots of variables, but they basically fall into four different categories; three that are physical and one that is financial/regulatory. FWIW, large commercial vessels (of any material) typically are designed to have an economic life of 20-25 years, and their economies are set up that way. The last payment on the vessels financing should be exactly the scrap value.
1) The Hull: Again comfisherman has it about right. With todays' modern coatings and regular maintenance a well built commercial aluminum hull will last the owners working lifetime; so 30-50 years with good inspections keeping ahead of problems. This assumes that there is always money in the pot for these inspections/maintenance. If this is a commercial business venture, a bad year or two with no money for maintenance, hull life could be shorter. Well built and maintained public aluminum vessels can have significantly longer lives due to front loaded funding. Note when I say commercial, I mean a vessel designed for 30+ years of daily operations, not made by a company for weekend sportfishermen. There is a finite life of thin skinned aluminum hulls. There is something called an S-N curve for metals; S is for stress and N is for the number of cycles. The more a boat is used, the larger the N and therefore the lower the stress that can be accommodated by the hull structure. Light highly loaded aluminium hulls will eventually fail in fatigue (generally at the welds which is why a lot of small aluminum boats are riveted); however, a vessel designed for commercial (i.e. day in and day out) use will avoid this. Google "s-n curve for aluminum" if you want to look into this more.
2) The Prime Mover: A lot of commercial vessels lifespans revolve around main propulsion selection. Operational profile, MCR, fuel quality and costs, mean time between overhauls, etc. all drive engine lifespan and therefore economic viability of the hull. Running it hard and putting it away wet will eat into maintenance funds quickly; but buying powering margin and robustness is a real up-front cost that needs an economic analysis over the known/assumed power plant lifetime. The hull will generally outlast most modern marine power plants except the really heavy(ier), more robust low to mid speed diesels; and even those will need one or two total rebuilds over the hulls' life, so check the design of engine access of the hull or it may be too expensive later.
3) Wiring and Electronics: This is an often missed consideration by small operators. Wires in a marine environment have a fixed lifespan and the clock starts ticking the moment you peel it off the spool. Expect to replace portions of the wiring by the 30 year mark, because it will start to have weird shorts caused by vibration wear and jacket cracking just like a cars. This means that the hull has to be designed with access to the wiring in mind. Vibration and salt atmosphere also eats electronics and switches. Again this is something that can be mitigated with up-front hull costs for robustness (such as hull space/access for climate controls, air handlers, DRU's, etc.), but that 20 yo hull/tank fitting sensor that fails is most likely not going to be available anymore. So you are left with the choice of buying up-front spares or replacing the system/parts of the hull at least once.
4) Financial and Regulatory: You vessel or commercial trade could be regulated out of existence; this happens a fair amount with fishing vessels (often revolving around licensing the hulls). Or your insurance could skyrocket; in fact the older the hull gets (of any material), the more you should expect to pay in insurance for commercial operation and liability.
