How wild would these parties get in terms of loud music, substance (ab)use and sex (especially compared to today's parties)?

Well, it depends where exactly you're drawing the line for "early twentieth century". Late nineteenth and turn of the century? Not particularly wild nor particularly loud, and with substance use limited to maybe having a bit too much alcohol. The emergence of outrageous aristocratic parties in the vein you're thinking of is linked to the group known as the "Bright Young People": aristocrats, artists, and gossip columnists that scandalized Britain with louche parties and divorces in the late 1920s and early 1930s. To quote from D. J. Taylor's Bright Young People (an excellent source on this fascinating set):

In individual Bright Young People were chary of acknowledging their status, so the group as a whole defies instant analysis. It was at once too heterogeneous, too far-flung and at the same time too precisely located, too blatant and too interior, too promiscuous and yet too inbred. Its members ranged form the rich and aristocratic - Bryan Guinness on his marriage to Diana Mitford in 1929 was supposed to have acquired an income of £20,000 a year - to the downright disreputable. Some Bright Young People became successful writers, journalists or artists, while others plumbed the depths of drink, drugs and disappointment. They were much written about and much misrepresented.

While they certainly did other things, their focus as a "set" was their attendance at the parties other members of the set held. The best-known of the Bright Young Thing parties was the "Bath and Bottle Party", held at St. George's Swimming Baths in London in July 1928. Admission was invitation-only, and attendees were to wear a bathing suit and bring a bottle of something to drink. This sounds pretty tame to us, but to the generation of middle-class adults who knew a party as a social gathering in formal dress with cards, drinks, and dancing, it was absurd and in poor taste. Themed parties like this, or fancy-dress parties where people came in costume as Edwardians, athletes, characters from novels, etc., were written about in the newspapers, celebrated and condemned. They often saw gatecrashers or passersby pulled in; fistfights could break out between men and women, and some guests would drink until they passed out. Some use of drugs beyond alcohol, like cocaine, did happen, but the Bright Young writers who chronicled/memoir'd the time treated that as an extreme exception and not something that happened as frequently as the out-group accusers thought - the same thing was true of sex, with outsiders assuming that these fast young women were sleeping with the society men they were familiar with, but in fact the unmarried girls of the set tended to have pretty tame love lives until they married.

Would there be parties open to anyone (including peasants) or only to an exclusive group?

While I don't think "peasants" is a useful term at all for this time period, the answer is somewhat similar to the one above, in that it's not until the parties of the Bright Young People that someone completely unconnected to the social group giving the party would generally be able to gain entry, because the Bright Young parties could happen in a more public place, could involve drunk people moving from one location to another, and could have many guests of various social standings and ages and in various types of dress (so there were fewer shibboleths to make it clear that someone present wasn't meant to be there). Those who were invited were not always part of the exclusive core group - they weren't even all technically young! The coverage of the Bath and Bottle Party, for instance, included aristocratic matrons sitting and talking happily in the dressing rooms.

How long could these parties last?

Same caveat as before. I can't say exactly how late Victorian and Edwardian balls tended to be, but the parties of the Bright Young Things were notorious for lasting much longer, with guests sometimes returning home in the morning. So the answer (and my ability to give you more detailed information) really depends on what you might mean by that date range.