The two sentences you said are grammatically correct. However, I would say as follows as natural expressions in Japanese people. >"Keep going straight for about three minutes." >約３分間にまっすぐ行き続けてください。 ３分くらいまっすぐ行ってください。 ３分くらいこのまま歩いていってください。 ３分ほど～ We Japanese people have learned keep ~ing=~続ける as its direct translation. I guess however, its nuance in Japanese conversation is like the above. "for about xxx minutes=約３分間" Yes, I think this is also grammatically correct but to me, 約３分間 in conversation is kinda businesslike. (It's like instructions of manuals or something) >"Keep going straight until you see the library." >"図書館を見るまで、まっすぐ行き続けてください。" 図書館が見えるまで、the same as the above after that. Hope it helps.