You can say no thats too much work load, we're understaffed or its too tight of a timeliness for the results.

But understand the ecosystem. People make promises that arent entirely dependent on them to be able to deliver

If your boss promises something that will take 150% of you capacity for the week does it make any difference whether you put in 80% or 100%?

Business will take everything you give. Theyre bean counters will be always calculating when it costs more to hire and onboard a new dev than to let you take your time....