I’d like to use Manjaro for a bit longer period of time as my main OS for work (web development). I used it once for that purpose with XFCE, but maybe for about a month, and I liked it. Now, I’d like to use it again with KDE.
Since it’s a rolling release distro, would it be safe to let updates hang around for some longer time? I see people recommending updates once a week. Would I be ok if I ran updates once a month on a production machine?
It is maybe more likely you should pay attention and do some manual intervention.
Otherwise … yes, more or less.
I sometimes pull out an old netbook that hasnt had a reinstall since 8.10 … and update it just for fun.
(meaning there are months … or years … in between)
I dont know if I actually suggest this as a maintenance method, but it should be … ‘possible’.
Welcome to the forum! 
Your question is based upon the wrong premise. Manjaro does not get updated once a week, nor is there any other set periodicity to the update process.
Manjaro is a curated rolling release, which means that unless it concerns a high-priority update ─ e.g. an urgent security fix ─ updates are bundled together. This way, you don’t get updates every day or every couple of days as with many other rolling-release distributions, and the update process is actually more of an upgrade.
For the Stable branch, the updates are usually issued once or twice a month ─ sometimes three times, but that is rare ─ and even though it is often possible to skip an update, it is not recommended that you do so.
If for whatever reason you don’t want to update Manjaro more than once a month, then perhaps a rolling-release distribution is not for you, and then you might be better off with a fixed-point-release distribution. 