A timescale is the length of time something takes, or is planned to take, from beginning to end. If a boss asks 'What's the timescale for this project?', they are asking how long it will take and by when it will be done. It is a countable noun: a short timescale, a tight timescale, a realistic timescale, within the timescale we agreed. There is a second, larger use in science and history: geological timescales, the timescale of evolution — here it means the whole stretch of time over which something happens, often unimaginably long. Two practical notes. It is much more common in British English than American, where people usually say timeline, time frame or schedule; and although timescale and timeline get mixed up, they are not the same. A timescale is the duration, an amount of time; a timeline is the sequence, what happens when, often drawn as a line with dates on it. British writers use both spellings, timescale and time scale, with the single word now more usual.