Once you accept the assignment, then I would devote as many hours as it takes to complete the assignment. I would examine the assignment and gauge how long it would take me to complete it in a way that I’m proud of the result. Then, I would decide if the company is promising enough for me to spend that amount of time on it. If yes for all 5, you have to see if you have enough free time to do that for all 5 companies! If not, I wouldn’t accept all 5. I would try to negotiate my way out of the bottom 2 assignments.
I would speak to the recruiter or hiring manager, and tactfully and politely tell them that I am really interested in the company and would love to continue the interviewing process, but due to personal and professional commitments, would not have time to do the assignment at this time. Of course, they might not want to continue the hiring process at this point (I wouldn’t do it to a Google or an Amazon). But you’d be surprised, I actually did this once to a startup and they totally just waved the assignment. They were like, don’t worry about it. I still went in for in-person interview, but no at-home assignment. Half the time they don’t really know what they’re doing with these assignments. ;)
To your strategy of accepting all of them and allocating a “max” number of hours: be careful… Say you think they each will take 5 hours. I’d only accept all of them if I had 30 hours to spare before the deadlines. 5 for each project, plus 5 for wiggle room because design always takes longer than you think. If you do not have the 30 hours, I would not allot 3 hours “max” for each of them or 4 hours “max” for each. You’ll just stress yourself out and you won’t feel good at the end. I would not go there.
I think designers should feel empowered to try to negotiate their way out of these assignments or to at least negotiate flexibility in the timing.
Does this help answer your question? If you’d like to provide more specifics on your situation, I’d be happy to provide more guidance!