Each organism has its own time, but the WHO (World Health Organization) unequivocally claims that radiation from gadgets is classified as carcinogenic class 2B (page 14), this class of carcinogenicity leads to oncology. For more details on the effect of a mobile phone on the occurrence of oncology (cancer), see the link.
The question is not formulated quite correctly, and therefore it is not completely clear what its author had in mind.
1) If you are asking about the relationship between mobile phones and brain tumors, then the world's largest oncological associations are not have a consensus on this score.
A working group of the International Agency for Research on Cancer in 2011 officially concluded that the use of mobile phones has a "potential carcinogenic effect." At the same time, the American Cancer Society and the National Institute of Environmental Health Sciences concluded that the available scientific evidence does not allow for unambiguous conclusions.
The FDA The US Food and Drug Administration ( FDA ) found that research had failed to prove a link between mobile phone exposure and health problems. The same is the opinion of the U.S. Centers for Disease Control and Prevention and the Federal Communications Commission. According to their data, the relationship between cell phone use and the development of cancer has not been scientifically proven.
In 2015, the European Commission Scientific Committee on Emerging and Newly Identified Health Risks also concluded that the available epidemiological studies do not indicate an increased risk of developing brain tumors, as well as head and neck tumors and other neoplasms in both adults and children.
However, doubters are officially recommended by the FDA reduce the time of calls on a mobile phone, do not "hang on the line" while dialing, and use wireless headsets.
2) Next. If you are interested in exactly how long you need to sit in a mobile phone, then you can dream up a little on this topic. You can imagine that people "sitting" on the phone lead a sedentary lifestyle, and as a result have problems with being overweight (there is a small, but interesting research on this). Obesity, in turn, is a risk factor for numerous diseases, including breast cancer.
3) And, finally, a direct answer to your question. Taking into account that the most significant risk factor for developing cancer is age (about 66 years is the average age for various types of cancer), then you need to sit in a mobile phone for about forty years.