Following Apple’s (AAPL) announcement this morning that more than 25 billion apps have been downloaded from its App Store for the iPhone, iPod Touch and iPad, ISI Group’s Brian Marshall, who has a Buy rating on the stock and a $525 price target, writes that he estimates the company’s cumulative shipments to date of devices based on its iOS operating system probably totaled about 315 million in the December quarter, up from 165 million in the year-earlier quarter.

Marshall makes reference to a note he put out on November 15th of last year, in which he argued Apple’s revenue is secured in the same way as the cable companies, with a high recurring aspect as users buy a “$650 high-end ‘set-top box’ with an ~18-month replacement cycle,” as he characterized it.

Hence, he estimates that the “implied value per iOS user” right now is $1,900.

Similar remarks were made by Toni Sacconaghi of Bernstein Research, last month, though he arrived at different “lifetime values” in his calculations.

Apple shares today are down $8.94, or 1.7%, at $536.24.

Fin