When does the letter 'e' have a long sound before a consonant? For example:
And in the case of Pelias, why can 'e' have both a short sound and a long sound?
These are fairly obscure words (except perhaps senior and Venus). The usual reason for any pronunciation in English is "because it is" and you have to get used to it. The deeper answer is the history of words changing sound without the spelling following.
The general reason for the letter "e" being pronounced as /iː/ is the great vowel shift in which the stressed or long "e" /eː/ changed to /iː/. In the case of "senior", "venal" and "Venus", the word is influenced by the French accented vowel, "sénior", "vénal" and "Vénus". The accented vowel sound becomes a long /iː/ when shifted in the great vowel shift. The word "menial" and "penal" seems to have gone down a similar route, originally "meinial" and "peinal", with a vowel change from /ei/ to /iː/, and and a spelling change too. "Pelias" is the same here the "e" is used to represent the greek letter "epsilon", but there has been a sound shift. Not a common word - I needed to look it up to check the pronunciation. The native Greek would be closer to /e/ than /i:/. I can't find any words that are even slightly commom that have a -melia suffix, the closest was "camelia", a type of flowering shrub.
That may have been interesting, but it won't help you decide if "genus" "Juvenal" or "plenary" have a long or short "e" sound (The first two don't and plenary has both pronunciations)
I don't disagree with most of James K's answer, but I think he's missing a point which needs to be made.
In Old English and Middle English, vowels in stressed open syllables tended to be long and in closed syllables short. (The distinction then, as far as we know, was really one of length, as in some other languages today. The distinction between so-called long and short vowels in English today is mostly the substitution of a different vowel sound).
In writing, a closed syllable ended with a consonant; and if another syllable followed, that started with a consonant. So there was a strong tendency for words with long vowels to be written followed by a consonant and another vowel, and short vowels to be written followed by a consonant and the end of the word, or by another consonant.
When the Great Vowel Shift happened, as James K says, the long vowels waltzed around the mouth, but the short ones didn't.
The upshot is that usually (though not always) a vowel letter followed by a single consonant and another vowel letter is pronounced with the so-called "long" vowel sound of Modern English. This is familiar in the case of "silent e", but it is more general than that.
I'm dubious that the pronunciation of "senior", "venal" and "Venus" owe much to French, as they follow regular rules of English pronunciation. Conversely, I suspect that "Pelias", known at least in modern times mainly from Debussy's opera "Pélias et Mélisande", has been influenced by the French pronunciation.
I can't account for "genus" (or "general"), or the "short" pronunciation of "plenary". "Juvenal" doesn't seem relevant to me, because the stress is on the first syllable.
Edit (months later): actually, I can account for "general" and "plenary" (but not for "genus"): these can be explained by trisyllabic laxing.
E may be "long" before a single consonant letter (other than x) that is followed by a vowel letter. But it isn't always long in this context. Even similar words may have different pronunciation patterns, and some words or names, like your example of "Pelias", may have multiple pronunciations.
Words ending in the Latinate suffix -al tend to follow a rule based on the position of the stressed syllable: if the stress falls on the second-to-last syllable, the vowel in this syllable will take its "long" pronunciation (unless there is a heavy consonant cluster after it). This accounts for the "long e" in venal and penal.
Nouns or names that have the exact same spelling in English as they do in Latin, like Venus, also tend to follow this rule about penultimate stress corresponding to a "long" pronunciation of a vowel letter that has only a single consonant after it.
The rule based on the position of the stress does not apply to all English words, only to specific types of words. For example, the word lemon (from French) has a short, not a long "e" in the first, stressed syllable, despite its spelling. The words metal and petal also are pronounced with "short e": despite being spelled with the letters "al", they don't contain the suffix -al, so they don't follow the rule mentioned above.