The nicest thing about being a freelance writer is that I get to make my own schedule. If I find a good opportunity for temporary work, I can arrange my schedule to meet my needs. If I want to do volunteer work, again I can shift certain projects to other days and do the extra work. Last week, I found myself dressing the Broadway musical "Wicked" simply because one of the dressers broke her foot and I was able to go in on less than 24-hours notice and finish the week.
That said, it is very important to complete your projects on time. If you have bid for one month to complete a non-fiction book and you have to wait on interlibrary loan to get the books you need for research, always contact your employer in advance and explain the situation to them. Yes, this has happened to me, but thankfully it is not often. Most of the time, the employer is slow to fund SafePay or something along those lines and that slows the process down.