For numbers of seconds, is it smaller or equal to 1 we use second, larger than 1 we use seconds?
SOME of the rules around 1 are:
"X somethings" when X is not 1
For 1 and 0 amounts with decimals pronounced "0 point Y" and "1 point Y", it is somethings:
0.5 somethings, 0.1 somethings, 1.5 somethings, 1.1 somethings
For quantifications ending on a something, we have half a something, a quarter of a something because it is still relative to 1 (or a)
The same is the case for time, weight, money and other quantifications.
a quarter of a second/kilo/dollar - note a quarter (dollar) is one coin in the US.
For the rest of the rules and exceptions and possibly perceived rules, have a read of the answers to Is -1 singular or plural?
We use the singular when there is exactly one. When there is more than one, even if it's just a fraction more, we use the plural. So "one second", "two seconds", "one and a half seconds", "1.4 seconds", etc. By the way, whether you spell out the numbers or use digits, the convention doesn't change.
When there is a fraction less than one, there are two common ways to say it. One way is to state the fraction and use the plural, for example, ".5 seconds" (pronounced "point five seconds") or "two-thirds seconds". This is more commonly used in technical writing. The other way, often used in more casual speech, is to say "[fraction] of a [thing]", and use the singular for the "thing". For example, "two-thirds of a second", "a quarter of a gallon". This second form is most used with ratio-type fractions, that is, we would say "1/10 of a second", but people rarely say "0.1 of a second". As an odd special case, with "half" we often omit the "of": "half a gallon", "1/2 a day" rather than "half of a gallon" or "1/2 of a day".
Zero always uses the plural: "The elapsed time was zero seconds."