In general, if you're going for authentic Roman numerals, you'd have to convert the decimal portion into one of the fractions that a Roman would use – or a sum of those fractions. Obviously, this is somewhat more straightforward for something like '1.5 hours' (for which there's also the single word sesquihora) than for '12.34' – though, for most people's purposes, 12.34 is close enough to 12-1/3 that they would probably just call it XII et tertiam (partem).
If the decimals are less straightforward, and/or if a very high degree of accuracy is required, Frontinus's work on aqueducts from the first century CE provides one example of how a sum of fractions can be used. In book 1, sections 38ff., Frontinus describes the diameter, circumference, and capacity of various pipes in terms especially of halves, thirds, sixths, twelfths, twenty-fourths, and two hundred eighty-eighths, using either the words for those fractions or a set of symbols for them.
For example, here's Frontinus's description of a fistula quinaria and a fistula senaria, using symbols:
fistula quinaria diametri digitum unum = –, perimetri digitos tres S = = – ℈ III, capit quinariam unam.
fistula senaria diametri digitum unum semis, perimetri digitos IIII S = £ ℈ II, capit quinariam I = = – ℈ VII.
- Twelfths: = is 2/12, = – is 3/12, and = = – is 5/12.
- Halves: S is 1/2.
- Twenty-fourths: £ is 1/24.
- Two hundred eighty-eighths: ℈ II is 2/288, ℈ III is 3/288, and ℈ VII is 7/288.
Therefore, a translation would be:
'Number-5' pipe: 1 digit plus 3⁄12 in diameter; 3 digits plus 1⁄2 plus 5⁄12 plus 3⁄288 in circumference; it has a capacity of 1 quinaria.
'Number-6' pipe: 1-1/2 digits in dimaeter; 4 digits plus 1⁄2 plus 2⁄12 plus 1⁄24 plus 2⁄288 in circumference; it has a capacity of 1 quinaria plus 5⁄12 plus 7⁄288.
This system works well for Frontinus and lets him achieve a remarkably high degree of accuracy. For example, for the circumference of a fistula quinaria, the total of the fractions given by Frontinus, when converted to a decimal value and rounded to eight decimal places, is 3.92713334. If pi is used, the diameter of a circle with a diameter of 1.25 units, rounded to eight decimal places, is 3.92699082. So he's within less than 2/10,000 of a unit.
Here's an image of the quoted text, in case your device doesn't show the symbols correctly:
And here's a version of the text that uses full words instead of symbols:
fistula quinaria diametri digitum unum, quadrantem; perimetri digitos tres, deuncem, scripula III; capit quinariam unam.
fistula senaria diametri diametri digitum unum, semissem; perimetri digitos quattuor, bessem, semunciam, scripulum; capit quinariam unam, quincuncem, sicilicum.