The initial lease term is 15-20 years, each time extendable by 5-10 years.
He promised to extend the initial term within three to four weeks from the signing date.
For indicating ranges, use a hyphen ( – ) not an em-dash ( – ) (a wide type of dash). Note that dashes are used to emphasise a phrase – or word – as opposed to commas or parentheses (which explain or add something without necessarily giving it emphasis).
18) When a range is indicated by a dash, do not repeat the symbol or multiple if they do not change. Close up the dash between the figures.
5,000-6,000, 5-6 percent, 5m-6m (not 5-6m) and 5 billion-6 billion.
In running text, use to. For example, provided that sales increase from 5m to 6m and estimates range between 5m and 6m. So do not write 5m-6m in either case.
If the symbol or multiple changes, however, leave a blank space on either side of the dash and preferably use an em-dash. For example:
100 kW – 40 MW
Where to would be part of a ratio, spell it out. For example, The General Meeting decided, by nine votes to two, to put the matter to the general assembly which voted 27 to 19. Where a ratio is used as an adjective, figures and hyphens may be used, but only if one of the figures is greater than ten: thus a 50-20 vote, a 19-9 vote. Otherwise, set the figures in words and use to (as well as hyphens because it is a ‘grouped adjective’). For example: a five-to-two vote, a ten-to-one probability.