My mother said, 'It's about time to start getting dinner ready.'

Literal

Mother [topic-は] soon dinner [possessive-の] preparations [object-を] do time is-[emphasis-よ] [quotative-と] said

そろそろ = 'it's about time / soon.' ~時間ですよ = 'it's time (to do X)' with sentence-final よ for emphasis/assertion. The mother's speech is quoted directly.