Not knowing how much time it takes to finish a task?

So this happened a while ago during my SA internship and now I’m approaching the start of FT I hope to get some advice on this one situation that I imagine would repeat later in work.

What happened was, I got asked to audit a comp database that contains 100+ transactions. I needed to check all the information from deal announcement date to available financials like ebitda and update with newly released info if I find any. I was quite slow probably because I wasn’t familiar with where all the info is located. I guess the associate got worried at my speed and one day he called me into his office and asked me whether or not I can finish the job by the deadline, which’s 2 weeks later.

I wanted to say yes because I had finished checking more than half of the transactions but I was also assigned to help a live deal just before the conversation so at the time I said “could you let me check my workload again and report back to you because I’m not sure at the moment”, and my associate was slightly upset hearing that and asked me that how come I’m not sure.

I was told later by another associate that I should never say “I’m not sure/I don’t know” since one can always estimate based on their past progress. I decide to follow that advice but I’m still afraid of giving out misleading deadline expectation when I myself have a hard time estimating it like when it’s something new or multiple projects all at a time. What would you do in said situation?