How long should It take to build a mf model from scratch?

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Totally depends on if you are building monthly or annually. And how much detail. For example, are you going to want to build loan covenant testing in, interest rate adjustments if the loan is IO for 3 years at 4.5% but amortization beginning year 4 is at 5.5%, fixed vs floating, choice for 30/360 vs Act/360 interest rates, etc. If you’re doing annually, and literally from scratch, probably 2-4 hours chances are if you have a development model as an interview round, you will have that amount of time. And they won’t expect you to build it monthly. 

As I said above, probably 2-4 hours. Many firms do take home tests. So you actually have 1-3 days. I’ve only once actually gone into an office to take a test. 
 

as a note, I had one condo development model that I was given 4 hours but they allowed it to extend to 7 (on a Saturday) because it was so complicated. 

I've never had a development model test.  I've interviewed for Multifamily focused PERE 50 firms.  1-hour to 2-hours is usually the time limit, with no waterfall models.  So far all the waterfall models have been take homes.

I built about 10 Tabs from scratch on a timed take home in about 3/4 hours, having done it before and knowing almost to a T exactly what it should look like. Here are the tabs I used for a Multi-Family renovation plan without a Waterfall. Keep in mind I had to read an OM and be able to defend my assumptions. I basically backed into Broker assumptions changed some and then spent the last hour writing a 2 pages on what I thought. This was experienced position, so it was less about modeling and more about thinking about your inputs with just an OM given.

1. Inputs 2. Expenses 3. Rent Roll / units to be upgraded 4. Upgrades per month and Expected Revenue 5. Taxes (Knew the market) 6. Debt 7. Cash Flows 8.Summary Cash Flows (Comparing T-12/T-6/T-3 to PF 1 and then PF 2345) 9. Summary Page 10. Deliverables (Answering Case Questions) then moved this to the first tab.

But to be fair my first job in the industry, I didn't know shit. I had to build a Multi-Family revenue model with Yr 1 Revenue given Expenses given and be able to model growth with an exit cap rate. I managed to get it right, but I looked back on it and it was complete shit. 

I had a really good mentor and ever since that job I have only been told I smoked everyone in the modeling portion. I wouldn't worry about modeling skills in your first job, your boss is going to want it all to look his way anyway. Font, Font Colors, Highlight Colors what is bold what isnt, ect. He is going to seem like an asshole, but eventually after having all those excel tips burned into your brain you will be able to produce IC ready content from scratch in a few hours.

These models you are making for interviews are just to make sure you understand the concepts. I am doubtful anyone would use them on the job, the interview isn't going to care that you have something that looks different, but more that you understand the concept and you are someone they can teach.

Built out a basic waterfall model, probably put in 5 hours total. As someone else above mentioned, I have not seen a monthly model, I feel like that could be a bit excessive.