Responsa for Berakhot 4:1
תְּפִלַּת הַשַּׁחַר, עַד חֲצוֹת. רַבִּי יְהוּדָה אוֹמֵר, עַד אַרְבַּע שָׁעוֹת. תְּפִלַּת הַמִּנְחָה עַד הָעֶרֶב. רַבִּי יְהוּדָה אוֹמֵר, עַד פְּלַג הַמִּנְחָה. תְּפִלַּת הָעֶרֶב אֵין לָהּ קֶבַע. וְשֶׁל מוּסָפִין כָּל הַיּוֹם. רַבִּי יְהוּדָה אוֹמֵר, עַד שֶׁבַע שָׁעוֹת:
The morning prayer may be recited until midday. R. Yehudah says: Until the fourth hour. [For, according to the Rabbis, the daily burnt-offering may be sacrificed until midday; and, according to R. Yehudah, until the fourth hour of the day. "Until the fourth hour" is until the end of the fourth hour, a third of the day when the day is twelve hours long. And its time is always until the end of a third of the day according to the relative length or shortness of the day, as stated above in respect to the Shema. And the halachah is according to R. Yehudah.] The afternoon prayer may be recited until the evening [i.e., until it gets dark.] R. Yehudah says: Until midway through the day (plag haminchah). [The time of minchah ketanah ("the small minchah") is from nine and a half hours until nightfall — two and a half hours — so that plag haminchah, which is half of that, is an hour and a quarter (before nightfall). The ruling in this regard is that one may follow either practice. If he wishes to follow the sages and to recite the afternoon prayer until the evening, he may do so, so long as he does not recite the evening prayer at that time. For since he considers it day as far as the minchah prayer is concerned, he cannot consider it evening for purposes of the evening prayer. And if he wishes to follow R. Yehudah, to recite the afternoon prayer only until the plag haminchah, an hour and a quarter before nightfall, he may do so; and from that time on, he may recite the evening prayer.] The evening prayer has no fixed time. [Its time is the entire night. It is taught: "it has no fixed time," rather than: "its time is the entire night," for the evening prayer is optional, corresponding, as it does, to the time of the consumption of the limbs and fat-pieces — the entire night. And the latter is optional; for once the blood has been sprinkled, the offering is accepted, even if the limbs and fat-pieces become unclean or are lost. Today, however, it (the evening prayer) has been accepted as binding.] And the mussaf ("additional") prayer may be recited the entire day. [If he delayed reciting it until after the seventh hour he fulfills the obligation, but he is called "an offender." And this is the halachah.] R. Yehudah says: Until the seventh hour.