Think or Thinking
When we use the verb, "to think," we can imply opinion or consideration by using either the simple or continuous tenses.
Simple tense - I think
If we want to express our opinion or belief, we don't usually use the continuous.
We can express this in the past, present and future.
"He thought the presentation could be better."
"I think that's a fantastic idea."
"What will she think about it?"
Continuous tense - I'm thinking
If we want to expression consideration or contemplation, it is preferable to use the continuous.
We can also express this in the past, present and future.
"He was thinking of changing careers."
"She is thinking of resigning from her job."
Present perfect continuous:
"I have been thinking about it."
"I will be thinking about it."
Both "thinking" and "think" can mean "to contemplate" or "to reflect on."
"I've been thinking about it, I think about it all the time."
In the present perfect, it is more likely that it's about consideration rather than opinion.
"I have thought about it for a while."