Until when can I say tehillim on a regular winter night?
Regarding saying Tehillim at night, according to the Mishna Berura it is permitted to say all of torah shbal pe at night and it is only preferable to say it during the daytime. According to the Arizal though it is stricter. Nighttime for this means from when it surely night, meaning from tzeis hakochavm until alos hashachar. However even according to the Arizal, it is permitted to say Tehillim at night, if it is being done as a tefilloh, for a sick person for a shidduch or as a shmira, such as the four perakim of Tehillim that as said before going to sleep, or the other pesukim that re said during kriyas shema al hamita. Therefore most probably you can say your Tehillim a whole night.
Mey Yehuda O:CH 22, Shar Hatzion 238-1, PiskeiTeshuvos 238-3.