What will happen if I don't have surgery for my colloid cyst?

Depends on cyst size. When the cyst becomes large enough, it can cause hydrocephalus. This can be life threatening in a rapid fashion.
Depends. Any symptoms yet? For asymptomatic cysts - you may never become symptomatic. Cases of sudden death have been discussed but are highly infrequent. Ask your neurosurgeon who can give you more specific instructions - based on your individual films and clinical exam.