answer to when is about half a second after you hand the money over.
Why? Because you can now dive whenever you want. If you have the opportunity to make a last minute dive and the dive shops are doing open water training that weekend and have all their tanks out, or are closed early in the morning, you don't have to go there to get tanks. You don't always have shops around like mentioned earlier, or you want tanks that suck less than al80's do for actual diving... Upkeep on tanks is pretty easy... $25 ish for hydro every 5 years, and about $15 each year for a VIP. Average steel tank lasts about 50 ish years of good use, and will usually fail vip before hydro. So call it $20 a year to have your own tank for maintenance, and say $80 for a used aluminum tank.
$2 difference here between renting and filling tanks, so if you dive the tank more than 10 times per year and the maintenance cost just paid for itself. Dive 40 times over the course of the tanks life and it just paid for itself.
I dive a lot more than that each year, about 150-200dives a year and I couldn't do it without my own tanks...
All that being said, tanks should be the LAST piece of equipment you buy as they are not really portable unless you can drive and everything else is more important