Well, as far as I can tell...
The official timeline is: Eva's Death • Manga • DMC3 • DMC1 • Anime • DMC4 • Novel 2 • DMC2
•Arkham's story about a woman who was murdered 10 years prior to the manga, along with the two flashbacks in the manga, suggest that Eva died 10 years before that.
•The manga is a year before DMC3, going off one of Dante's comments in the game.
•Dante is a teen in DMC3. Late teens from the look of him.
•Trish states that Dante "lost a mother and a brother to evil 20 years ago" in DMC1. Trish could well be rounding up or down, placing it anywhere between 17 and 22. But it's equally likely that actually IS 20 as it is any other number. Plus, the idea of Mundus creating a demon to look like Eva to manipulate Dante on the 20th anniversary of her death sounds like something he might do. Possibly a stretch, but since 20 is the number we're given, I'm inclined to say it's 20, placing this 9 years after DMC3. He is also "about Cobra's age", which is 29.
•Assuming that Vergil is Nero's father (that's the timeframe they originally planned the game with), Nero was conceived during Vergil's lifetime. Maybe not born during it as he could have been conceived less than 9 months before DMC3. Since Vergil would presumably spend more time with Arkham as their plan grew closer to fruition, my guess is that Vergil visited Fortuna shortly after first meeting Arkham in the manga, possibly at Arkham's suggestion (echoing Arkham's daughter telling Vergil's brother about Fortuna). If Nero is born close to DMC3, and is DMC3 Dante's age during DMC4, then DMC4 Dante is double that age. This means DMC3 has to be old enough for DMC4 Dante to be "nearly 40" (between 37 and 39).
•The anime is some time between DMC1 and DMC4. The DMC1 outfit suggests it's closer to DMC1 than DMC4. Trish has had enough time to get sick of Dante and leave, so I'd say at least a year since DMC1. Lady also needs time to fill out to her DMC4 proportions.
•The second novel's epilogue leads directly into DMC2, while also stating that "several months" have passed since the main story of the novel.
•DMC2 is set "far after DMC4" according to Kobayashi.
Eva's Death - +10 - Manga - +1 - DMC3 - 9 - DMC1 - +? - Anime - +? - DMC4 - +? - Novel 2 - +>1 - DMC2
With that rough timeframe, we now have to work out the ages of the characters.
•DMC3 Dante is a teen as stated previously. Assuming Vergil did indeed sleep with a prostitute to produce Nero, I'm sure Capcom would want to avoid any unfortunate implications, and with their intense desire to appeal to the American market, I would guess they would want Vergil to be an adult in the eyes of Americans. All U.S. states set age of consent to 16-18, so 18 seems a good call, since it also allows Dante and Vergil to still be teens in DMC at 19, and few places if any set age of consent above 18. This is important because if the twins were Japanese, one could argue cultural differences. They are not, and are assumed to be American, so following American age of consent is a good idea.
•Assuming these ages hold true, using what we have so far, we now have:
Eva's Death - 8
Manga - 18
DMC3 - 19
DMC1 - 28
DMC4 - 38
•This gives us a 10 year gap between DMC1 and DMC4. Using my previous logic, the anime presumably falls within the first five years of that decade, but at least 1 year after DMC1. Since Lady also needs time to fill out, it probably doesn't take place close to the middle of the decade. Given the notable difference in Lady's appearance, I'm willing to place the anime at 1-2 years after DMC1.
•"Several months" doesn't suggest much time has passed, so Dante is probably the same age in both.
•Finally, all we know for DMC2 is that it's "far after" DMC4. But look back over the ages so far. Notice a pattern? Something notable occurs every time the last digit of Dante's age is 8. If this pattern holds, Dante in DMC2 could well be 48 years old.
So with that, my final conclusion is this:
Eva's Death - 8
Manga - 18
DMC3 - 19
DMC1 - 28
Anime - 29/30
DMC4 - 38
Novel - 48
DMC2 - 48
I've had this timeline mapped out in my mind for a while now, but I've yet to see anything that contradicts this. Does anyone have anything to add or dispute with this (the order of events doesn't count).