my mom still has a papier mache elephant I made in the 5th grade.  That was 18 years ago at least.

Hi, PM has been used for hundreds of years and some pieces can still be found in any good antique store in the form of tables, chairs and dressers etc.
Every piece I sell has a card attached with instructions on the maintenance of pm pieces whether for in door use or occasional out door use. There isn't a lot of work involved but like any wooden piece of furniture or carving, occasionally pm pieces will need say... a coat of varnish for example to protect it from chipping especially if the piece is in regular use or used out doors. The more ware and tare a piece gets will determine how often a piece will need maintenance. Some of my pieces only require it every few years or so.
Good luck with the picture frames, my only other piece of advice would be to make sure your work is well sealed to prevent moisture getting into them. Some people may decide to hang them in areas like a bathroom or kitchen where condensation can build up. It only takes a little crack to allow moisture to get in, and then the integrity of the piece will be quickly threatened.

does linseed oil help with the longevity of the mache, or is that just for preventing mould?

That is the million dollar question.  I tried linseed oil (painting it on sanded, unvarnished 'raw' Strip/pasted Paper mache until the piece couldn't "drink" anymore 3 or 4 times) I had a terrible time trying to prime and finish the object.  What I discovered later is that if you are going to linseed oil it, you have to let it 'cure' over several weeks or months, depending on the climate.  The pieces I made with linseed oil are the strongest and most durable of all of my pieces.  They seem to grow harder as they age.  The paint has never chipped and they stand abuse much better than my other pieces (I have a bunch of boys AKA 'The Mache Mashers') who love testing and retesting each and every one of my PM pieces by putting them through  series of random tests and adventures.  They have tested my linseed oiled pieces repeatedly and have yet to mar the surface of one.  If I wasn't so impatient, I would probable oil all of my strip/pasted pieces.~tammy wilson