I am a conlanger and I would like to know when PGmc was spoken for a new Germanic/Italic conlang I am making.
Roughly, 500 BCE - 200 CE.
There's no firm date at which Proto-Germanic stops being Proto-Indo-European, or becomes other Germanic languages. These are all artificial divisions imposed on a continuous process of evolution.
However, when people talk about Proto-Germanic, they generally mean the last common ancestor of the modern Germanic languages, which has been dated to somewhere around 500 BCE. To quote Ringe, the best authority I know on the subject:
Proto-Germanic [...] is unlikely to have been spoken before about 2,500 years ago (ca. 500 BC).
[...] This is not the only possible definition of PGmc [Proto-Germanic]; for instance, the 'Protogermanisch' of Euler 2009 is a somewhat earlier stage of development of that language.
[...] Early Jastorf, at the end of the seventh century BC, is almost certainly too early for the last common ancestor of the attested languages [...]
Similarly, the end of Proto-Germanic is generally said to be when different dialects have changed significantly enough to be analyzed on their own. The oldest inscriptions of any Germanic language are Proto-Norse, written in the Elder Futhark script; for example, the Øvre Stabu spearhead, dated to the second half of the second century CE.
However, not everyone considers Proto-Norse meaningfully distinct from Proto-Germanic. There's reasonable evidence of Proto-Norse from the second century onward, but no comparable evidence of other dialects, so we can't compare them to see when they become meaningfully different. The major changes which separated Old Norse from other attested languages (the three different types of umlaut) didn't happen until after 500 CE. So even if you don't consider Proto-Norse an independent language, that's a definite upper bound on the end date.