On 7 Cheshvan they start saying "tein tal u'matar" in Eretz Yisrael. In chu"l we don't start until December, when the rainy winter season starts outside of EY. There is a whole debate in poskim regarding what a ben EY who is visiting in chu"l should do and vice versa, so ask your LOR what to do if you will be travelling.
For those in EY, there is an interesting question as to whether one should start saying tt"u if one davens maariv early, after plag, on the night when we switch over. Why should it be different than, for example, davening maariv on leil Rosh Chodesh after plag, where one says yaaleh v'yavo even though it is not yet night? The reason we make the switch to teil tal at this point is because Chazal tell us that it took 15 days for the last person to make it back home from their aliya la'regel on Sukkot. We don't want to start asking for rain while people are still on the road, as it would make travel difficult. The recitation of tt"u is therefore dependent on the # of days that have elapsed since Sukkot, not the switch from day to night or a switch in kedushas ha'yom. One might therefore argue that until actual nightfall when 15 full days of travel time have elapsed one should not change to add tt"u.
The language of SA in OC 117 would seem to support this position. When it comes to the din in chu"l, the SA says
וּמַתְחִילִין לִשְׁאֹל מָטָר בְּחוּצָה לָאָרֶץ בִּתְפִלַּת עַרְבִית שֶׁל יוֹם ס׳ אַחַר תְּקוּפַת תִּשְׁרֵי
SA uses the term תְפִלַּת עַרְבִית -- whether it is davened after tzeis or after plag.
However, with respect to EY, the SA says
וּבְאֶרֶץ יִשְׂרָאֵל מַתְחִילִין לִשְׁאֹל מִלֵּיל ז׳ בְּמַרְחֶשְׁוָן
Here SA uses the date of לֵּיל ז׳ בְּמַרְחֶשְׁוָן. It is the calendar date, which does not switch until nightfall, rather than the time of maariv, which is the determining factor.
Whether this is the maskana l'maaseh I'll leave to others to decide.
