wochenlang
for weeks
Lasting over a period of several weeks.
Die Renovierungsarbeiten dauerten wochenlang.
The renovation work lasted for weeks.

for weeks / weeks-long
Loading your saved state.
for weeks
Lasting over a period of several weeks.
Die Renovierungsarbeiten dauerten wochenlang.
The renovation work lasted for weeks.

The word is made up of 'Woche' (week) and 'lang' (long). It describes something that lasts as long as several weeks. The '-n-' is a linking element that often appears after feminine nouns ending in -e.
Die Suche nach der vermissten Person dauerte wochenlang.
The search for the missing person lasted for weeks.
'Wochenlang' (for weeks / weeks-long) can be used as an adverb that describes the duration of an action. It can also stand before a noun as an adjective, but then it has to be declined.
Wir haben wochenlang auf eine Antwort gewartet.
We waited for weeks for a reply.
This structure with '-lang' is very common for describing periods of time. You can also form it with other units of time such as 'Tag' (day), 'Monat' (month), 'Jahr' (year), or 'Stunde' (hour).
Die Kinder spielten stundenlang im Garten.
The children played in the garden for hours.
weekly
'wöchentlich' and 'wochenlang' both have to do with weeks, but they mean something completely different. 'wöchentlich' says how often something happens: once a week. 'wochenlang' says how long something lasts: several weeks, without the focus being on short individual appointments. Learners may confuse the words because they look similar. Pay attention to the question: 'How often?' goes with 'wöchentlich', while 'how long?' goes with 'wochenlang'.

Nach dem Umzug suchte sie wochenlang nach wichtigen Unterlagen.
After the move, she searched for important documents for weeks.
Der Arzt kontrolliert die Werte wöchentlich.
The doctor checks the values weekly.