It depends on the system and filters, how often the water system is used and what's in the water being filtered. Each system and location will be a little different. There are some good guides, though.
1) In a typical home RO system, the first filter is a 5 Micron sediment filter. This filters out sediment of 5 microns (5/1000th of a millimeter) or larger.
2) The second filter depends on if your water source is municipal or not.
A) In a home RO system that uses municipal water, a Carbon Block Filter is needed. This filter is specially designed to take as an method in removing chlorine and its by-products (TTHMs) and volatile organic compounds, and certain organic compounds both natural and man-made, among other things.
B) If you're not on municipal water, the second filter should be a 1 micron sediment filter. This removes more than the 5 micron filter, but because the 5 micron filter gets used first, it experiences a longer lifetime and is able to focus on finer particulate that are more common to untreated/well/local water sources.
3) The third filter should be a GAC (Granular Activated Carbon) filter, which removes harmful chemical compounds through adsorption. The very high surface area of the carbon in these filters provides excellent filtration for chlorine, and harmful organic compounds.
4) The membrane is the workhorse of the Reverse Osmosis system. It's where the main, fine filtration happens. By creating a zone of pressure that forces water through the membrane, very fine particles are filtered out, leaving clean, safe drinking water.
5) Post storage tank inline GAC filter. This will be the last filter in the line (second last if you have a remineralization filter, but most people don't) before the water goes to the faucet. It filters the "rubbery" taste from the storage tank, and has an average lifespan of 2-3 years.
Removing the tube in order to replace membrane can be a little tricky if you're not familiar with colletts.
The collette that keeps the tube in the fitting works sort of like a finger trap. If you push the collette in with a fingernail or flat edge of a butter knife, or pliers, or a similar tool, you should be able to pull the tube out. Sometimes a little twisting can help if it's stuck even when the collette is pushed in.