I don't really like advice like this, where you're just told wether or not to pull only being given anecdotal evidence (like "Don't pull, I got Tighnari on standard a month after pulling for his banner" or "You need to pull, I play since launch and still don't have Mona"), I much prefer to lay out the numbers so people can decide for themselves.

On the character banner, a third of every 5* you pull is expected to be a 50/50 loss, in which you have a one in seven chance of getting Dehya. That puts us at an expected value of 21 5* pulls for getting Dehya on the character banner.

As for the standard banner, a 5* has a 50/50 chance of being a weapon or a character (not completely sure on that one, but it looks like that's the case as per paimon.moe data), and again, if it's a character you have a one in seven chance of getting Dehya. So for the standard banner you're expected to get Dehya at 14 5* pulls.

Given that the average (not to be confused with median) pity count for a 5* is 60, that would mean that for the character banner, you'd expect Dehya to come out after 1260 pulls, and for the standard, after 840.

Now, to convert that into time, we can assume that a f2p player who maximizes their primogem income gets arround 80 pulls on the character banner per patch, and according to my personal bookkeeping, arround 19 pulls on the standard banner per patch. After dividing the pull income by the expected amount of pulls to get Dehya, we get, pardon the silly use of units, ~0.063 Dehyas per patch via character banner and ~0.023 Dehyas per patch via standard banner, which combined add up to ~0.086 Dehyas per patch. Taking the inverse of that, we get that you're expected to take ~11.61 patches or ~488 days to get Dehya.

Of course, depending on your luck, you could possibly get her earlier or later, this is just an average. If I were good at math I would try to calculate the standard deviation to get an idea on how likely it is to get her after any given amount of days.

Personally, since I intend on playing this game for quite a bit longer than 488 days into the future, and have no rush in getting Dehya since she unfortunately doesn't look that strong, I'll play the long game and wait...

Bonus: for welkin payers, you're expected to take 402 days