I have had shoots for ca. one year without transfer, they can stay alive but of course not in the best state.

Its not only the sucrose, but also think about the nutrients (both macro and micro, and others) being depleted from the medium due to growth.  Therefore, it will depend on the growth rate (depletion rate) volume of medium (available source at the beginning of experiment) etc.  The usual practice is to subculture every 4 weeks or so, but that can vary due to the above factors.  You can monitor the growth (visually) over time and see how long you can go without subculturing.  If you are looking at extending subculturing interval, you can also reduce growth temperature, a common trick used in germplasm conservation practices.

Did you mean 'shoot' (no roots) or 'plantlets (with roots)? It depends on the plant species, for tobacco shoots (no roots, we cut the shoots and stick it into medium for rooting), it can just sit there for several months without growing roots. For tobacco plantlets (with roots), it can really sit in the boxes for a long time, until the medium dry out. I have seen some plantlets sitting there motionless in the boxes/plates even the medium is completely dried-out. Of course, the leaves are a little bit wildered compared to the good ones. When they reach this status, if you don't do anything, they will eventually die. But, if you add just WATER into the plates, they will 'wake up' again. And when they 'wake-up' again, move them to the soil, they can grow as good as you can expect.

thanks for the nice answers.

In addition to potential nutrient depletion, accumulation of ethylene in the culture vessels should be considered on how often we have to sub-culture. The smaller the volume of the vessel, the more frequent ones should sub-culture. Moreover, the more sensitive the cultured explants to ethylene exposure, the more frequent ones should sub-culture. Accumulated ethylene in the culture vessels may cause senescence to the cultured explants. I hope this can add some info to the previous answers.

every parameter which hels in explant response matters. in general the transfer should be done around 5-6 weeks. if the media dries out the shoot needs to b transferred or else the culture will be stressesd out...

I kept cultures (multiplication stage) from strawberries, Alnus spp., birch for one year in the fridge. They had to be regrown once a year on fresh medium at 23C; when growth was resumed, they were placed back in the fridge for another year. I kept cultures for up to 5 years without problems (after 5 years, I threw them away since I was not working anymore with these species).

Yes there was no light and the refrigerator was REALLY at 4 degree C.  Most people don't check their fridge but everybody says "4 degree C" because that's what is supposed to be. However, if you check with a thermocouple, fridges can vary from minus 1 (and you kill everything) to even + 10 degrees depending on the fridge and on the shelf within a fridge. So, check your refrigerator. If your fridge is around 8, your cultures will suffer much faster and won't preserve  as long as they would have at 4.

You are very welcome.

It depends upon plant to plant  but normally 15-20 Days is good enough

Of course it depends on the plant.

In my growthroom I could keep cacti sterile on agar medium even for 5 years.then finally the medium dried away even with the good closure of parafilm. That is being 16 h under light and in 25 Celsius.

But of course that was not your question. The interval of transplantation depends very much on your idea of the outcoming result.

For e.g. you can keep unrooted shoots for 6 to 8 weeks on 0.5 MS and voila they root on theit own.