Stratification doesn't necessarily mean that the seed is going to germinate inside the ziplock bag inside the fridge. Most of the time it doesn't. After the stratification period inside the fridge sow the seeds in a container and put it outside or in a green house.

Sometimes the problem is a thick seed coat that prevents water to reach the actual seed (for instance Acer griseum / paper bark maple). Scarification might be helpful with these seeds but be careful not damage the actual seed (usually leads to rotting of the seed). Also: based upon my own experiences, for some species stratification in the fridge does not work very well and these can better be sown in a container outside (Stewartia among others).

Temperature for stratification: most guides say about 5C/20F but 10C / 50F will do for many species.

Length of stratification: you do indeed get different advice when it comes to the length of the stratification period. I like to sow my seeds before Christmas(most outside, some in the fridge). It means that they get 2 to 3 months of cold stratification. It usually works.

Note that some species first need a warm period before a cold period (Fraxinus, Chionanthus, Paeonia, Styrax and Stewartia for example).

Many species can be stored for some years without significant loss of viability. There are however species that can't be stored for very long or should be stored under the right conditions to maintain viability. The problem for the seed buyer is that he/she cannot see how old a seed really is. There is no easy solution to this problem. All I can say is: buy only from reliable sources and share your experiences with others.

Thanks! How cold does it get in your place? And wonder too weather the seeds have to be kept moist at all the time - in nature it is sometimes dry too. I think outside here is simply too warm even when they say + 5C, climate is different here even we get frosts.
In the fridge do you use coffee filters or sand?

I live in the south of the Netherlands and usually we have mild and wet winters and only in some winters get severe frost (more than -10 C / 14F). Lowest temperature this winter so far was -6 C / 21 F. I would say that day time winter temperatures lies between 1 to 10 C. / 33 to 50 F. so not very cold.

I don't have to water my seeds that are outside in winter because there is very little evaporation in winter. Sowing compost should be moist but not wet. For seeds that are outside I use a well draining sowing compost. I've added extra sharp sand (aka builder's sand) to improve drainage.
I use the same sowing compost for seeds that are in ziplock bags in the fridge. If you squeeze the sowing compost no water should be dripping from it (but the compost must feel moist).
The containers with seeds that are outside are covered with a thin layer of fine gravel. Not only does this prevent drying out of the sowing compost but it also helps against weeds and mosses (liverwort is the worst).

I've added a picture of my seed frame. It's covered with shade cloth to prevent birds etc. having a go at the seeds.

Loewenzahn, you might find this helpful:

http://www.gcstl.org/_ccLib/attachments/pages/Seed+germination+instruction.pdf

Thanks! The document is helpful.
I find the gravel interesting. Maybe it helps the seedling when it is very wet? In your climate it gets colder and as it is more towards the pole - it is colder for longer hours. Here it dips only to -5C during the night and that is at most, usually maybe -2C or so and during the day you can work outside in a jumper.

The gravel helps to prevent a crust from forming on top the pots if it dries, and a heavy rain (or watering) from dislodging the seeds. It can help prevent moss and liverworts from forming on the top of the sowing medium with those things that have a longer germination time, and help seeds to make good soil contact without excluding all light if they need that.

I generally use chick grit from the feed store to top my pots. Larger seeds are covered with some of the sowing medium or potting soil, then topped with the grit. Tiny fine seeds are sown on top a light layer of grit and rinsed down with a quick spray of water.

If I'm out of grit and can't go to the farm supply, I've used parakeet gravel from the pet section of the supermarket. Fine aquarium gravel is often available by the pound if a pet/fish supply shop is convenient to you.

You might want to read through the sowing techniques on the Clothiers Site, lots of good information there. You could benefit from bookmarking the site for the seed germination databases too....invaluable.

Here is a link that might be useful: Clotheirs