How often do you replace your shower curtain liner ?

Mine has two layers cloth and plastic from dollar store, I machine wash both curtains once a month, wash in 'handwash mode' so the plastic doesn't rip. I change the plastic curtain once a year, less plastic in garbage.

Inside the tub I use a mat with suction cup, it's safe and easy to scrub with a sponge. Not sure of your parents age but providing scrubber with long handles will be easy from them to keep tub clean.

Outside tub I use micro plush memory foam bath mat, it has a rubberised base.

I scrub my bath mat with a brush and disinfectant when I clean my tub. The shower curtain liner gets replaced when it starts growing mildew in it. It’s only like $4 for a new one (less if you want a really crappy one). I’d rather spend a few bucks every few months to buy a new shower curtain liner than try to clean it. I just spray it down with disinfectant and rinse it when I clean the bathroom but it will eventually get gross. I probably change it a couple times a year. 

I buy good quality merchandise to avoid having to replace things any sooner than necessary.

My bath mat and shower curtain liner sre about 5 years old. I wash and dry them in my washer and dryer. My bath mat does not go in the shower, it goes on the floor outside the shower. I wash that weekly and my shower curtain liner gets washed every month or so.

If your room mate wants to spend money on things, that is his choice.

That new tub mat (it's not a bath mat, because those are a type of rug that is used on the floor of the bathroom outside of the tub) is a safety device that you shouldn't be critical of. Bathtubs can be and often are very SLIPPERY, and one of the most common causes of serious accidents at home are falls in the bathtub. Accidents like this are particularly dangerous for elderly people, because they can lead to tragedy.

Properly installed and cleaned regularly, a tub mat will last for years. It's important that it be cleaned and disinfected regularly, however. This prevents athlete's foot and other health concerns, and also helps to stop the spread of viruses, such as those which cause colds and flu.

How often a shower curtain needs to be replaced depends on what kind of curtain you have. The ones made from plastic or vinyl only need replacement if they are allowed to get moldy- and mold is something that is easy to remove with the right cleaner and a little elbow grease. If you have a cloth shower curtain, however, it will need to be washed regularly. Most of those are generally machine washable, but follow the manufacturer's directions when it comes to cleaning.

Every few months when it looks gross. They are $5-10, so not expensive.

what decade is this? So, "Maybe" once in 10 years? Maybe 20? So like never for the mat.

  The shower curtains if they be the plastic type, they fade or discolor not really much else. so 10 years or after the last fad has been forgotten. Clowns or dinosaurs or Star Wars.  They just hang there.  Most of the time they do not rip, really depends on the ANIMALS YOU GOT IN THE HOUSE. Drunken pigs will tear them out in a fortnight. Party hearty somewhere else dude as I KEY YOUR CAR only because a hammer was not available.

Lord--how much could a shower curtain liner and bath mat actually COST you? I'd replace them as soon as they got so moldy and stinky they were not cleanable. That's probably about every six months or so--if you keep them clean, they last a lot longer. If your roommate thinks yours are gross, let her or him replace them. 

every time I fall-over and RIP-IT-DOWN ..................

Every few years, some quicker than others.  There's point where the mildew doesn't seem to completely wash out of the curtain anymore and it seems to get moldy again overnight almost.  It's similar with the bath mat, especially when the texture starts to change.  At that point I start to dread my ablutions.  The space doesn't feel clean anymore so they need to be changed.  "... so bad" kind of implies that they were not nice just not "bad enough" to your mind. People have different tolerance thresholds.  I knew a lady who didn't find out that sheets could be removed from the bed and washed until she started college.  She only realized when she saw others do it and so copied them in order to fit in but she never saw the point herself.  I've also shared a house with someone who threw out the toaster, electric kettle and microwave every single year because that's what her parents did.  As I said, different tolerances.  Most people aren't so extreme.

It is time to replace when they are falling apart or when you fancy a change.  Shower curtains can be washed and so can mats. I don't have a curtain any more as I have a screen, but when I did, if it was a fabric one, I would put it on a quick wash in the washing machine.  For plastic curtains and the floor mat, I use bathroom spray cleaner and rinse.